A box contains 60 apples. If one-fifth of the apples are rotten, what percent of the apples are rotten?

One-fifth of 60 apples is (1/5)60 = <<(1/5)60=12>>12 apples.

The percentage of rotten apples is (12/60)*100 = <<(12/60)*100=20>>20%. Answer: \boxed{20}.