REM sleep is the phase of sleep when most active dreaming occurs. In a study, the amount

Question:

REM sleep is the phase of sleep when most active dreaming occurs. In a study, the amount of REM sleep during the first four hours of sleep was described by a random variable T with probability density function if 0 <t< 40 1600 f(1) = if 40 <t< 80 1600 20 otherwise where t is measured in minutes.
(a) What is the probability that the amount of REM sleep is between 30 and 60 minutes?
(b) Find the mean amount of REM sleep.