Below is the question. Please help

2. EEE 350Random Signal Analysis A professor schedules two student appointments for the same time. The appoint ment durations are independent and exponentially distributed with mean 20 minutes. The rst student arrives on time, but the second student arrives ve minutes late. The second student has to wait if the rst student has not departed when the second student arrives. What is the expected value of the time between the arrival of the rst student and the departure of the second student