What is the answer 11+11=4

12+12=9

13+13=?

Ok! Nice question, by the way!

11 + 11 = 4

What is the sum of the digits of 11? 1 + 1 = 2.

Now square that. You get 2^2 = 4.

12 + 12 = 9

What is the sum of the digits 12? 1 + 2 = 3.

Now square that. You get 3^2 = 9.

Now, 13 + 13 = ?

What is the sum of the digit 13? 1 + 3 = 4

Now square that. You get 4^2 = 16.

13 + 13 = 16.