Last night I spent the evening with some of my family who live nearby. I cooked a big pot of chicken and sausage gumbo, and we had a fantastic time catching up.

As I sat down on the couch after devouring a sugar cookie for dessert, my uncle handed me one of those big legal note pads. On it was a brain teaser that a friend had sent him via email. In my sugared brain fog, I worked through it and thought I would share it with you.

Look at the following series of numbers and determine which number goes in place of the question mark.

2,3,10

3,4,21

4,5,36

5,6,55

6,7, **?**

You can post your answers in the comments below, and then check the solution to see if you were correct.

If you enjoy these teasers, please share them with your friends and family by clicking the Blue Share Buttons below. I appreciate your support!

tom says

the total of first number times it self and the first number times the second number eq5uals the third number. i.e. (3*3) = 9 + (3*4)= 12 = 21 then (6*6)= 36 + (6*7) = 42 = 78

Swans says

Answer is 78

OldNassau says

Differences of 11,15,19, so 23 + 55 = 78.

Could be posted above.

Daniella says

78

Nancy says

answer should be 8

Barbara says

x,y,z

formula= z/[x+y]=x

6,7,z

equation= z/13=6 -_switched_around_- 13*6=answer or a

So answer or a=78

Anonymous says

78

ArmchairFlyer says

78

Lori says

78

Jack Parker says

‘(a+b)*a’

6,7

13*6

78

Anonymous says

78

Anonymous says

78

Anonymous says

252

Anonymous says

67

Lori says

78

Coretha says

my answer is 78

Coretha says

the answer is 78

liz mcneill says

78

Anonymous says

78

liz mcneill says

the answer is 8l

nimesh says

( 6 + 7 ) * 6 = 78

mayukh majumdar says

the difference of 1 on each row remains the same.But the first number is multiplied with a difference of 2.so 6*13=78

Anthony H says

There is other ways to get to 78 also.

The second number multiplied by the addition of the next rows numbers.

Example –

2,3,10 <- take the 3

3,4,21 <- add 1st two numbers 3+4=7 multiply by the 3 (7 X 3 = 21)

4,5,36 <- 4 + 5 = 9, (9 X 4 = 36)

5,6,55 <- 5 + 6 = 11, (11 X 5 = 55)

6,7,78 <- 6 + 7 = 13, (13 X 6 = 78)

shakeba pink says

My answer is 78

mary @2 says

According to me the answer is 78 and I got it by the following logic:

3×7=21

4×9=36

5×11=55

6×13=78(that is all the odd no. from 7)

Humna Hanif says

78

q-boom says

1. 2,3,10 (2x(2+3=)5=10)

2. 3,4,21 (3x(3+4=)7=21)

3. 4,5,36 (4x(4+5=)9=36)

4. 5,6,55 (5x(5+6=)11=55)

5. 6,7,? – 6x(6+7=)13=78

answer=78

Ratasha says

I came to 78 as the answer. Looking at the other methods, I looked at patterns. 2,3,10

3,4,21

4,5,36

5,6,55

6,7, ?

I saw that the first and third numbers had a commonality; 2*5=10 3*7=21 4*9=36 5*11=55 so 6*13=78. Not the mathematical genius as others have used but it yielded the same result :)

Ashford says

78

EQUEENS says

It’s 78! Each column has a pattern.

ala2_naiem says

the answer is 78

I found it in another way :)) I sum the first line with the second line without the last no. then i subtract 1 like 2+3+10+3+4-1 = 21 so 5+6+55+6+7-1 = 78

Satwant Hundal says

Another Answer is

2,3,10 –> (2*3)*2 – 2=10

3,4,21 –> (3*4)*2 – 3=21

4,5,36 –> (4*5)*2 – 4=36

5,6,55 –> (5*6)*2 – 5=55

6,7, ? –> (6*7)*2 – 6=78

Miles k says

78, cause I’m good the pattern is 2+3×2, 3+4×3, 4+5×4 and so on and so forth

robert reed says

2,3,10 2+3=5, 5×2=10

3,4,21 3+4=7, 7×3=21

4,5,36 4+5=9, 9×4=36

5,6,55 5+6=11, 11×5=55

6,7, ? 6+7=13, 13×6=78

answer ?=78

mrwram says

78

yrhyck says

78

tom says

78

tom says

30

jack says

it’z 78..

Jette says

Instead of trying to find a formula that could incorporate all the numbers given, I only looked at the last column of numbers (10, 21, 36, 55, and the unknown number. I calculated the differences between each value, and got 11, 15, and 19, which all have a difference of 4, therefore i added 4 into 19, and added that into 55. The unknown value that i calculated was 78. :)

21-10 = 11

36-21 = 15

55-36 = 19

15-11 = 4

19-15 = 4

19+4 = 23

23+55 = 78

? = 78

Anonymous says

we got same answer but different ways.just like

2,3,10 – [ 2*5]= 10

3,4,21 – [ 3*7]= 21

4,5,36 – [ 4*9]= 36

5,6,55 – [ 5*11]=55

6,7, ? – [ 6* 13]= ? =78

i think of odd ( 5,7,9,11,13 )numbers and multiple them thats how i came up with the answer..

Anonymous says

Thats exactly what I did! And got 78

Anthony H says

That’s the same way i did it. But yea there’s obviously more ways to do it but that’s definitely the fasted.

jennie says

i’m thinking its 58

charlotte says

6,7,78

kit says

78

Fati says

the answer is 78

this is how I got to it:

the last number of each line is calculated like this:

let’s assume that the first number is a and the second is b, and the last is c

then we would get that c on all the lines :

c=(a*b)+(a*a)

so the last line goes like (6*7)+(7*7)=78

Ethel says

The answer is 78. 2+3=5 and 5 goes into 10 2 times. 3+4=7 and 7 goes into 21 3 times. the numbers climb up times 4,times 5, and times 6. 6+7=13×6=78

Bcc says

78

lili says

78, you must check the third line, i mean 10+ (11)=21, then 21+( second odd numeber after 11 i mean 15) now you have=36 and again 2 odd num after 15 again i mean not 17 of course 19 now we have 55 here is look: (36+19=55)

55+( second odd after 19( i mean 23))= 78!!!!

lili says

78

nully says

there is more than 1 way to solve it..

but i got to the same answer..78 ^_^

al bebert says

the answer is 78

choolwe says

78

vidhi says

78 its easy :D

sobia says

78

Anonymous says

You take the product of the first two numbers and add it to the square of the first number to equal the last number. For example, 2,3,10:

you multiply 2 by 3 to equal 6. Then you add the 6 to the square of the first number which in this case would be 4 (2 squared).

Sandy Sheridan says

79

palie3 says

78

Ane says

a, b, c

c = (a + b) x a

6 + 7 = 13

13 x 6 = 78

angelica martinez says

2,3,10

3,4,21

4,5,36

5,6,55

6,7, ?78

paige says

78

2×5=10

5+2=7

3×7=21

7+2=9

9×4=36

9+2=11

11×5=55

11+2=13

13×6=78

thana says

the correct answer is 78 (10+11)=21 (21+15)=36 (36+19)=55 (55+23)=78

al2gator says

I think Mandy’s remark about adding 4 to the sequence is by example:

21 – 10 = 11. Add 11 + 4 to 21 and you get 36.

al2gator says

What I liked about this puzzle is that there are a couple of patterns that work:

Many in the comments mentioned (a+b)*a and one noted the final numbers add 4 each time. But what I liked is that (a*b)+(a/b)(a*b) also works.

Hrisi says

the answer is 78 Nice sequence.

Pranathi says

ans:78 its pretty easy but its gud.

Kevanna says

the answer is 78,, it really wasn’t dat hard

Tabassum says

78

JUSTICE says

78

Nnanna Osuji says

The number is 69

Moises Sanchez says

The answer is 78. Add the two numbers, then multiply the total by the first number.

dina says

78

Carolyn says

88

Evan Bageris says

78

Phil says

78

manmeet says

78

ann March says

I made it 78 too

boss says

78

may anne says

78.. .

Tanya says

Was A Bit Harder Than I Though! I Thought Maybe 2+2+3+3 = 10 So Tried The Others But Failed :D Got It Eventually =] …

2+3=5 *2=10

3+4=7 *3=21

4+5=9 *4=36

5+6=11 *5=55

So… 6+7=13 *6=78 :D

Julie Guillemette says

78

juneil says

78

pragati says

78

Anonymous says

easy stuff… 78

April Mae says

78

Mandy says

Actually you can just add 4 to the last number in each sequence. Answer is still 78.

Alex says

I got the pattern in less than 10 seconds but screwed up multiplying (6+7)*6. Got 252 after multiplying 6*7 and again by 6, lol.

nadiege says

the answer is 78.

catadeluxe says

78, I got this very fast, I’ve noticed the 10, and the 21, and then, all the things came into my mind

Makua says

The answer is 78. I feel so smart. lol

taher abdo says

78

samar says

78

music=life says

The answer is 78 because the pattern is:

2+3=5*2=10

3+4=7*3=21

4+5=9*4=36

5+6=11*5=55

6+7=13*6=78

the answer has to be 78

Brenda says

78

Michelle says

78

LoLi says

78

miya says

i got the answer 78 tbh wasn’t that hard i was clearing out my inbox and i was going through my old emails when i found the teaser. The pattern was pretty clear :) 6 + 7 = 13 * 6 = 78

Tammy Balbinot says

78

Shuaib says

2+3=5. multiplying this with 1st term(2) we get 10.

likewise 3+4=7, 7*3=21

4+5=9, 9*4=36

5+6=11, 11*5=55

6+7=13, 13*6=78

so the answer we got is 78

Susan says

69

aeon says

pattern is first value + second value = result X first value

6+7 = 13 (6) = 78

sue says

2+3=5*2=10

3+4=7*3=21

4+5=9*4=36

5+6=11*5=55

6+7=13*6=78

so the answer is 78.

AINE says

The answer is 78

Boris says

2+3=5;5*2=10

3+4=7;7*3=21

4+5=9;9*4=36

5+6=11;11*5=55

6+7=13;13*6=78

kim says

?=73

Sandi Weiso says

2+3=5, 5*2=10

3+4=7, 3*7=21

4+5=9, 4*9=36

5+6=11, 5*11=55

6+7=13, 6*13=78

so, ?=78

Louise says

78

Anita says

2*3=6+(2*2)=10

3*4=12+(3*3)=21

4*5=20+(4*4)=36

5*6=30+(5*5)=55

6*7=42+(6*6)=78

Therese says

78

vaibhav says

The answer is 78.

Shakespril says

60

pam says

answer is 78

Gill says

78 (6+7= 13 X6)

Antonella says

23

Evelyn says

78

Selwyn Simons says

It took me about 20 seconds to figure it out 78 is the answer.

Jose Izaias says

Hi everybody!

I did twice:

first:

same as everybody.

second:

2, 3, 10 — 6 + 4 ==> (2×3=6) and (2×2=4)

3, 4, 21 — 12 + 9 ==> (3×4=12) and (3×3=9)

4, 5, 36 — 20 + 16 … same …

5, 6, 55 — 30 + 25

6, 7, 78 — 42 + 36 ==> (6×7=42) and (6×6=36)

gdsfg says

78 1min

Geraldo says

78 4 minutes

Christine says

78….the thing that I have found interesting is the number of different methods by which people have arrived at the answer. The pattern I saw was add the first two digits then multiply the answer by the first digit, but others have arrived at the same answer by many different paths.

Erin Matlock says

I thought it was interesting too, Christine. I solved the teaser using your solution, and it is so cool to see the ways that others figured it out. :-)

HALI says

I WAS RIGHT!

abgroove says

78. Geez, I’m slow on the draw on this one.

Carol says

78…

april says

78

Robin says

74?

Malka says

It was too easy!!

Dont think about the small numbers they are useless

Its + 4 between each result

21-10= 11 + (4) = 15

36-21= 15 + (4) = 19

55-36= 19 + (4) = 23

After that just do 55 + 23 = 78

wene69 says

The answer I got was 78 as well. The pattern I saw was subtracting the first right hand number from the one above it gave an answer that increased by 4 each time going up the line.

Wayne.

maya says

it’s is definitely 78

ravi says

78

therese says

I think everyone who needed to say how fast and/or how easy they found this need to ask them- selves “how important is it”????,

Erin you have the patience of a saint, when you read through these replies?

therese says

I mean’t you MUST have the patience…….. I don’t know if I could deal with all this “I need to be the best and fastest” attitude every day, Kudos Erin

Robin says

I was the one who asked about speed, and it isn’t an ego thing in my case. It is important to me to understand my own intelligence versus processing speed because I have a non-verbal learning disability and a high IQ. It has to do with my own journey. There’s nothing wrong with a little friendly comparison – there are millions of people who are quicker and smarter than all of us. I wasn’t racing when I was doing the brain teaser, I just happened to glance at the clock and note the time. From the responses I read 4 minutes is a tad slow, but I already suspected as much. So, for me, it is important, but for most I think it’s just a bit of fun :)

~Robin

Erin Matlock says

The great thing about this place, and the thing that keeps me going, is the diversity. We are all dealing with different circumstances relating to our brains yet we share the common ground of wanting more from them – or at least to maintain our current level function.

I did not time myself, although I remember being frustrated that I didn’t see the pattern sooner. I like to see other people’s times, because even though this wasn’t a contest, it gives us something to measure ourselves against (if we choose too.)

Therese, you come from the angle of just enjoying the process. This is something I am still learning to do. ;-) I know you represent a significant number here including many who chose to not post a comment or even email their answer privately. I thank you so much for being a part of this community.

Robin and the others who posted times (some of you were super fast!) represent those who were looking to share a bit more info about their experience. And, yes, we also have competitive folks here too. I love seeing it all, and I thank you for being a part of this community.

You all make this place interesting, and it is so nice to see a bit of your personalities. I hope to see more.

So many of you are more alike than you realize, and in the very near future, you will have the chance here to get to know each other better.Keep the answers coming! I will write up a post highlighting some of the different solutions that were listed in the comments.

therese says

I think I’m just fed up of trying to compete and compare. I’m pretty sure you can understand me when I say that life can be pretty lonely when one (for me cursed) has an above average intellegence. I have found it almost impossible to find people with whom I can truly relate. So in order to have some semblance of peace in life, I had to accept who I was and stop comparing myself to others. I was likely relating to how intense life was like, until I came to be able to ask myself “how important is it”, I did not mean to step on toes.

Anonymous says

This was a hard one. 78. I think I got it in under 10 seconds. Come on people, if you love your brain push it to the limit.

Anonymous says

whoops x’d it out incorrectly ought to be 78 :) Aly Swan.

Anonymous says

I’ll give it a go. Is it 69? It was a bit of fun doing this, thanks to you and your father. Aly Swan. x

Max Gash says

Surprisingly I figured this out in 30 seconds!

Robin, I don’t think that 4 minutes is slow. It’s where you’re at with this particular brain teaser. What’s important is that you focused on this brain teaser for 4 minutes until you figured it out. That takes healthy brain muscles!

jochrstphrr says

2 x 5 = 10

3 x (5+2) = 21

4 x (7+2) = 36

5 x (9+2) = 55

6 x (11+2) = 78 (answer)

Robin says

You guys are funny! No one will admit to taking longer than I did, which was 4 minutes. Oh well, I didn’t do too badly for not knowing a bit of algebra.

Erin Matlock says

Not too bad at all, my friend!

Andie says

78!! about a minute to figure out… including trying to find the calculator on my compute to multiply 13 by 6!! i surprised myself :)

therese says

I would be interested to know how many people had to actually use a calculator? I never use one myself, and thought that was at least 98% of the challenge of these brain teasers, I would be a genius in everything I do if I used props. And Erin,I texted this while smiling. That has been the greatest piece of info. you could have ever passed on to me, thanks so much, life has been a lot easier for me since you shared that.

Anonymous says

KEN

The answer is 78. It took less than 1 minute to figure out

David says

78 is the answer. It took me about 30 seconds to see the pattern (without the sugar fog :-)

Dave

jamie Parks says

198-20

Karen says

I got 78, but a different way- by ignoring the second number of each set, and progressing the multiplier by 2 each set.

2 x 5 = 10

3 x 7 = 21

4 x 9 = 26

5 x 11 = 55

6 x 13 = 78

jamie Parks says

4 Times 9= 36????? ot 26

Janine says

78. Less than 2 minutes.

shidzumom says

42

Afton says

answer is 78 6+7=13×6=78

JYO says

2+3=5*2=10

3+4=7*3=21

4+5=9*4=36

5+6=11*5=55

6+7=13*6=78

Anonymous says

obviously 78

Lori says

“78” I hope is the answer

I added the first two numbers together, then muliplied it by the first number.

6 + 7 = 13 13×6= 78

Charlene Barnes says

78

Ashish says

78

Erik Lindholmer says

My answer is: Given the 3 numbers a, b and c in all your examples it is so that c= (a+b)* a. The answer is therefore (6+7)*13 = 78. Nevertheless I cannot exclude other solutions. Erik Lindholmer

erno says

The answer is 78. the formula is 6 +7 = 13 x 6 =78.

Solution found in less than one minute!

Zayaan says

78…I hope…!…

Carla Constanza says

Answer is 78, and I must confess, I’ve seen that pattern recently, so figured it out in a jiffy.(Facebook site is “Life Begins at 50 Radio Show”.

Carla Constanza says

Whatever smartness I have does not extend to the technological. I was unable to navigate within the square to delete or edit, sorry.

David says

Answer- 78

The way I got the answer fast was to simply add 4 to it because that was what the pattern was telling me at first.

David says

Sorry, I meant from 11, 15, 19 and finally 23. From the pattern of 10, 21, 36, 55, 78. Then add 23 to 55 and you get 78. That’s how I did it.

Zayaan says

78…I hope..!…

Kariena says

78

winter says

78

2,3,10= (2+3)x2 = 10

3,4,21 = (3+4)x3 = 21

4,5,36 = (4+5)x4 = 36

5,6,55 = (5+6)x5 = 55

6,7, ? = (6+7)x6 = 78

Sholen says

78

Sholen says

79 – 1min

tanjen says

78

chelo says

answer is 78, 2 minutes

Kara says

?= 78

(2+3)*2=10

(3+4)*3=21

(4+5)*4=36

(5+6)*5=55

(6+7)*6=78

Felix Tonny says

Two possible routes to the solution!

Option one

21-10 = 11

36-21 = 15

55-21 = 19

? -55 = 19+4

therefore ? = 55+19+4 = 78

Robin says

78 – about 4 minutes. Is that slow?

Jesus Himself says

2 * 4826809 ^ (1/6) * (99 / 11) ^ (1/2)

Lawrence Silverman says

I found the answer by first noticing the pattern in the numbers on the right in that they increase by 4 each time. I’m not looking at it right now but recollection is that they went up as follows: by 11, then 15, then 19 and I rightly determined the next number would go up by 23. I then looked for a mathematical relationship that could be determined by the first two numbers and found what many of you found. It was neat to see that the right increase was X+4, while the left relied on a more cumbersome Xsquared + X times Y.

Sheila says

? = 78

Chris says

78

SRE says

78

> 21-10 = 11

> 36-21 = 15 = (11+4)

> 55-36 = 19 = (15+4)

78-55 = 23 = (19+4) <

Jier says

78

Mark says

78

bryna lee says

clearly (6+7) x 6 = 78 that was fun but pretty easy. not too much of a “tease.” Thanks!

Dennis says

2(2+3)=10

3(3+4)=21

4(4+5)=36

5(5+6)=55

6(6+7)=78

Would someone like to express this as a general algebraic equation?

Deedee says

a(a+b)

Mary Lemire says

I believe the answer is 78. The first two numbers added multiplied by the first.

katherine says

Yes, 78. that was easy but good !!!!!!!!!!!!!!!!

William, says

I come up with 78 and I see it aggrees with all of the above, or the majority of the above….

William, says

or you can say the sum of the first 2 #s x the 1st # = the 3rd #.

Josh says

Yeah 78

Didnt take me that long

Mohsin Aladin says

78! Easy Peasy Lemon Squeezy!

Erin Matlock says

Easy Peasy. One of my favorite phrases! :-)

Maria JosÃƒÂ© says

78

Maria JosÃƒÂ© says

I think 78 is correct

therese says

Ok to all the smarty pants who felt the need to elaborate that this brain teaser being “too easy”, I would like to remind you of the fact is that everything is EASY if you know how. I know for fact that had I not found this great site, a few months ago it would have likely baffled me. So again thanks for sharing this with us Erin

Erin Matlock says

LOL I clearly need to post a more difficult teaser next time. Duly noted my friends! :-) (Therese, you are too kind.)

Miss Lisa says

I usually have a hard time with puzzles, but found this one easy. I believe the answer is 78.

Terry Boehm says

whoops….78 is correct

Terry Boehm says

42

a=first number;b=2nd number;c=3rd

c=a(a+b)

therese says

in case I was not mean’t to say how I reached the answer, I’ll just say the answer is

78

Love your website Erin

Erin Matlock says

Thank you Therese! I appreciate it. :-)

therese says

the answer is 78 the first 2 numbers are added then multiplied starting at 2 for first line, 3 for 2nd, 4 for 3rd etc…

mitranee says

Hi,

My answer is 78.

Thanks

hali says

69

chris says

the answer is 78

you add the first two numbers together, then you multiply the added number by the first number.

RedRockJan says

Final answer 6 + 7 = 13 x 6 = 78

Ana Lisa says

252

Rob says

? = 78

Anonymous says

The answer is 78

Carol says

78

TofthS says

too easy, and I suck at these – 78

Qball says

78

boombada says

doh misstyped, i mean

(7+6)*6=78

boombada says

(5+6)*6=78, oops :]

SK says

its 78!!

Ron says

78

But oddly enough a variation of this puzzle is on my whiteboard at work :)

Erin Matlock says

Great minds think alike!

mondo says

78

Janice says

not sure how i ended up there..sorry

Erin Matlock says

No worries, Janice. You’re not alone. :-)

mohit says

78

c2london says

I agree, 78.

Robert says

78

wendy says

78. pretty easy i thought.

dee says

78 is the answer!

Helen Bound says

Answer=78

2+3 =5 x 2 =10

3+4 =7 x 3 =21

4+5 =9 x 4 =36

5+6 =11x 5 =55

6+7 =13x 6 =78 ta da!!

Kerri Cary says

78 this was an easy one if I got it…cause i’m really bad at number games…but thanks for the teaser

Aphrodite21 says

The answer is 78 and this was easy.

Stig says

6*(6+7)=78

Anonymous says

minh said 78

Jose Izaias says

78

suego says

252

suego says

i meant 78

Anusha says

78 it is..;)

lizardo says

78

Anonymous says

78

Anonymous says

Shoot, I wanted to be first:-( 78

Janice says

78

Anonymous says

its 78!!

Dave says

yep, 78. I concur with lizardo.

rach says

same

Parvati says

78 – a bit too easy even for innumerate me

Jane says

The answer to the brain teaser is 78 – 7+6=13×6=78