The length of time taken to learn a set of instructions may be treated as having an exponential distribution with a mean of 24 minutes. There is only one copy of the instructions, so as soon as one person has completed the learning process, the next person begins, with no intervening gap.
a) What proportion of those learning take between 20 and 30 minutes?
b) What is the 60th percentile of learning times?
c) What is the probability that at least three persons complete the learning process in a single one-hour period?