Paper 2 · November 2018 · permutations and combinations

What is the formula, in terms of n and r, for the number of permutations of r objects chosen from n distinct objects?

Model answer

n!/(n-r)!

Also accepted: n! / (n-r)!, nPr = n!/(n-r)!, nPr=n!/(n-r)!

Explanation

A permutation counts ordered selections. Starting from n choices for the first position and decreasing by one for each subsequent position gives n(n-1)...(n-r+1), which simplifies to n!/(n-r)!.

Derived from ZIMSEC Statistics Paper 2, November 2018, Q2

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