As of the writing of this post, 119 answers have been posted to last week’s brain teaser.

If you haven’t solved it yet, **don’t scroll down any further**. The answer is posted, along with some solutions from our readers.

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The answer is 78. Check out some of the solutions listed below.

*Winter posted:*

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

*Bryna posted:*

Clearly (6+7) x 6 = 78 that was fun but pretty easy. not too much of a â€œtease.â€ Thanks!

*Erno posted:*

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

*Erik posted:*

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)*6 = 78. Nevertheless I cannot exclude other solutions.

(Note: *Edited typo to illustrate the formula correctly*.)

*Kara posted:*

(2+3)*2=10

(3+4)*3=21

(4+5)*4=36

(5+6)*5=55

(6+7)*6=78

*Wayne posted:*

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.

*Malka posted:*

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

*Felix posted:*

Option one

21-10 = 11

36-21 = 15

55-21 = 19

? -55 = 19+4

therefore ? = 55+19+4 = 78

*Lawrence posted:*

I found the answer by first noticing the pattern in the numbers on the right in that they increase by 4 each time. Iâ€™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.

*William posted:*

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

**Do you have a favorite brain teaser you’d like to share?** Email it to us here, and we’ll post it on the blog.

ArmchairFlyer says

August 3, 2016 at 10:12 pmArmchairFlyer did pretty much the same as Malka, ignoring the first two numbers which were clearly in a simple series and looking at the last number of each entry as a series.

Sean says

September 25, 2012 at 12:40 amIf assign the position of the numbers with

these arbitrary variables:

a, b, c

then

a(sqaured)+a*b= c

a(sqaured) + b*a = c

2,3,10 4 + 3*2 = 10

3,4,21

4,5,36

5,6,55

6,7, ? 36 + 7*6 = 78

Ethel says

May 19, 2012 at 12:44 pmThe answer is 78. 2+5x(2)=10

3+4x(3)=21

4+5x(4)=36

5+6x(5)=55

6+7x(6)=78

The number in each equation is multiplied by the next number in sequence

ramsha says

December 30, 2011 at 2:30 pm78

Sagar says

December 23, 2011 at 9:36 pm78

Jeff says

April 3, 2011 at 3:27 pmthe first two terms are irrelevant. The final numbers follow the pattern that the first

10 = 2*5

21 = 3*7

36 = 4*9

55 = 5*11

?? = 6*13 = 78

Add one to the left increasing multiplier and two to the second.

d'zulia says

April 1, 2011 at 11:22 pm78

Georgios says

February 5, 2011 at 2:33 amits a^2+b^2-b=c

2^2+3^2-3=10

3^2+4^2-4=21

…………

6^2+7^2-7=78

Marwa says

January 26, 2011 at 2:19 pm78

Migz says

April 3, 2010 at 4:44 am2*3=6+2^2=10

3*4=12+3^2=21

4*5=20+4^2=36

5*6=30+5^2=55

6*7=42+6^2=78