Correct Answer: 8+11=96 

Solution: 1

If you look at first 3 Equations a + b means a + ab

a+b => a+ab

Let see now how it works for all equations ,

1 + 4 means 1 + 1(4) = 5
2 + 5 means 2 + 2(5) = 12
3 + 6 means 3 + 3(6) = 21

now will apply same formula for the last equation 8+11=?

8 + 11 means 8 + 8(11) = 96

So final answer is  96 is proved.

Solution : 2

1 + 4 = 5
5 + 2 + 5 = 12
12 + 3 + 6 = 21
21 + 4 + 7 = 32
32 + 5 + 8 = 45
45 + 6 + 9 = 60
60 + 7 + 10 = 77
77 + 8 + 11 = 96

Once again we get the answer of 96, that’s surprising!

Didn’t get 96? Check out the second way to solve the problem.

Alternate method : 

a+b= a+b(sum of the previous equation)

1 + 4 = 5.

2 + 5 (plus the previous sum of 5) = 12.

3 + 9 (plus the previous sum of 12) = 21.

same logic will apply for last equation also

8 + 11 (plus the previous sum of 21) = 40.