The Difference: nCr vs nPr
Choosing the right formula depends on whether the order of the selection matters.
Permutations (nPr)
Permutations are the number of ways you can arrange items where order matters. For example, if you are choosing a President, Vice-President, and Secretary from a group, the order matters because each position is different.
Combinations (nCr)
Combinations are the number of ways you can choose items where order doesn't matter. For example, if you are choosing 3 people for a committee where everyone has the same role, it doesn't matter which order you pick them in.