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20! Factorial of 20

Value of 20! i.e, Factorial of 20 =

In general, the factorial of a number N = N*(N-1)*(N-2)....1
Factorial of a non-negative integer n is the product of all positive integers less than or equal to n. Factorials are commonly encountered in permutations, combinations, number theory and series expansions of trigonometric and other functions (Taylor series).

Number of trailing zeros in 20! = 4
To understand factorials, you might find it useful to read the Factorial tutorial over here.