Twenty one
21 is an odd number.
21 is a Harshad number, which means it is divisible by the sum of its digits (2 + 1 = 3, and 21 is divisible by 3).
In binary, 21 is represented as 10101.
In hex, 21 is represented as 15.
21 is the 8th Fibonacci number.
Prime factorization of 21: 3 * 7.