11 + 11 = 4
12 + 12 = 9
13 + 13 = ?
You have to develop a system that explains that first two answers and then apply that system to the third equation. So this question might not have just one right answer...
You can create a system that for example adds up the digits of the numbers and then multiplies like so:
11 + 11 = 4
(1+1) * (1+1) = 4
2 * 2 = 4
12 + 12 = 9
(1+2) * (1+2) = 9
3 * 3 = 9
13 + 13 = ?
(1+3) * (1+3) = ?
4 * 4 = ?
4 * 4 = 16
Answer: 13 + 13 = 16