Test 1 Section 7 - #11 (page 417)

If they're telling us she has a 2/5 probability of picking a brown towel, we can think of 2/5 as the ratio of brown towels to all the rest of the towels in her closet.  Since they tell us she has 6 brown towels, we can set up a proportion to solve for the total towels:

Cross multiply to solve, and you'll see she has x = 15 total towels.