2, 3, 5, 7, 11, 13, 17,... what's the next number?
That's not so easy! The series is that of the prime numbers. A prime number is one that is divisible only by itself and 1. For example 21 is not a prime number (it is divisible by 3 and 7). After 17 the next prime number in sequence is 19.
Anyway, we checked your knowledge of math and your logic!