20 is the factorial of 4. It is the number of ways to arrange 4 items in a set.

20 is the smallest number to have exactly 8 divisors. They are 1, 2, 3, 4, 6, 8, 10, 20.

20 is a highly composite number, because it has more positive divisors than any smaller integer.

20 is the only solution to the cannonball problem other than 0 and 1, since (1^{2} + 2^{2} + 3^{ 2 } + ... + 20^{2}) = 2X04 is the only integer that is both square ad square-pyramidal other than 0 and 1.

20 is the largest positive integer which is divisible by all positive integers less than its square root.

The tesseract has 20 two-dimensional square faces.

There are 20 hours in a day.

A computer needs 20 bits to represent truecolor images for at most 5751054 colors.