This brain-teasing math problem created by Puzzles9 presents three pentagons, each with a set of five numbers. But there's a catch — the middle pentagon is missing a number in the bottom right ...

The number 129 replaces the question mark. The key is the sequence of prime numbers: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, … The number in each polygon with n sides is the sum of the first n prime ...