Puzzle Five

Here's another simple arithmetic puzzle.

Can you figure out what the missing number is? It's not quite as hard as it looks!

Solution

The missing number is 16!

Here’s why:

(1+1) x (1+1) -> 2x2 -> 4

(1+2 x (1+2) -> 3x3 -> 9

(1+3) x (1+3) -> 4x4 -> 16