Find the missing number?

21
18
23
14

The missing number is 14.

The numbers in the sequence are all the numbers from 1 to 24, except for 14. This is because 14 is a prime number, and the other numbers in the sequence are all composite numbers.

A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. A composite number is a natural number greater than 1 that is not prime.

The sequence is as follows:

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24