You own 9 pairs of jeans and are taking 4 on vacation. in how many ways can you choose 4 pairs of jeans from the 9? a.504 b.36 c.126 d.362,880

Since you don't care about order, we will be talking about combinatorics.
Combinatorics compare the total objects (n), with the objects that we care about (r), and follows this general rule:


Thus, let's substitute what we know. Our total number (n) is 9 pairs of jeans, but we only care about picking 4 pairs (r).
Substituting those in will yield:

different ways.