Find the missing number?

13
8
15
11

The answer is 11.

The numbers in the sequence are all the prime numbers between 1 and 20. A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. The first few prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, and 23.

In this sequence, each number is 2 more than the previous number. So, the missing number is 2 more than 10, which is 11.