20

Twenty (20) is a positive integer one more than 19 and one less than 21.

Properties
20 is an abundant, composite number. Its factorization is 2^2*5. The previous abundant number is 18 and the next is 24.

20, being twice the number base of 10, has sometimes been used as a base for naming numbers. For example, in french, 80 is quatre-vingt, which means 4*20. Also, 71 is 60+11, not 70+1 in french.

20 is the fourth tetrahedral number (between 10 and 35).