The distribution of number of hours worked by volunteers last year at a large hospital is approximately normal with mean 80 and standard deviation 7. Volunteers in the top 20 percent of hours worked will receive a certificate of merit. If a volunteer from last year is selected at random, which of the following is closest to the probability that the volunteer selected will receive a certificate of merit given that the number of hours the volunteer worked is less than 90 ? (A) 0.077 (B) 0.123 (C) 0.134 (D) 0.618 (E) 0.923

But why is it that the answer in the answer key is C? How to get the answer using the calculator? Thanks!