Paper 2 · 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
