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!
About the author, Erin Matlock
Facebook
Twitter
StumbleUpon
Digg
Email This
194 Comments On This Post
78
It took me about 20 seconds to figure it out 78 is the answer.
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)
78 1min
78 4 minutes
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.
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. :-)
I WAS RIGHT!
78. Geez, I’m slow on the draw on this one.
78…
78
74?
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
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.
it’s is definitely 78
78
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?
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
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
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.
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.
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.
whoops x’d it out incorrectly ought to be 78 :) Aly Swan.
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
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!
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)
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.
Not too bad at all, my friend!
78!! about a minute to figure out… including trying to find the calculator on my compute to multiply 13 by 6!! i surprised myself :)
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.
KEN
The answer is 78. It took less than 1 minute to figure out
78 is the answer. It took me about 30 seconds to see the pattern (without the sugar fog :-)
Dave
198-20
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
4 Times 9= 36????? ot 26
78. Less than 2 minutes.
42
answer is 78 6+7=13×6=78
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
obviously 78
“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
78
78
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
The answer is 78. the formula is 6 +7 = 13 x 6 =78.
Solution found in less than one minute!
78…I hope…!…
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”.
Whatever smartness I have does not extend to the technological. I was unable to navigate within the square to delete or edit, sorry.
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.
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.
78…I hope..!…
78
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
78
79 – 1min
78
answer is 78, 2 minutes
?= 78
(2+3)*2=10
(3+4)*3=21
(4+5)*4=36
(5+6)*5=55
(6+7)*6=78
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
78 – about 4 minutes. Is that slow?
2 * 4826809 ^ (1/6) * (99 / 11) ^ (1/2)
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.