21

Twenty-one (21) is a positive integer one more than 20 and one less than 22.

Properties

 * 21 is a composite number, factoring into 3*7.
 * 21 is the 8th Fibonacci number (between 13 and 34), and the 7th lucky number, between 15 and 25.
 * 21 is the 6th triangular number (between 15 and 28).