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 #.

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ArmchairFlyer says

ArmchairFlyer 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.

If 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

The 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

78

78

the 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.

78

its a^2+b^2-b=c

2^2+3^2-3=10

3^2+4^2-4=21

…………

6^2+7^2-7=78

78

2*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