Factorial
GMAT glossary · Quantitative Reasoning
Definition
For integer n ≥ 0: n! = n × (n-1) × cdots × 1, with 0! = 1.
Factorial in plain English
A factorial multiplies together every whole number from a chosen number all the way down to 1, and we mark it with an exclamation point. So 4! = 4 × 3 × 2 × 1 = 24. The real reason anyone cares: it counts the number of ways to arrange things in order, 4 different books on a shelf can be lined up in exactly 4! = 24 different orders. One quirky rule to memorize: 0! = 1, because there's exactly one way to arrange nothing (do nothing at all).
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