In a group of 120 people, 90 have an age of more 30 years, and the others have an age of fewer than 20 years. If a person is selected at random from this group, what is the probability the person's age is less than 20?

P(< 20) = [ 120 - 90 ] / 120 = 30 / 120 = 1 / 4

Thanks, CPhill.