Your computer checks for new email messages every 15 minutes

Your computer checks for new email messages every 15 minutes. If you sit down at your computer at a random time, what is the probability you will have to wait more than 5 minutes for the computer to check for new email messages?

Solution

Probability that you have to wait 1 minute for the computer = 1/15

Therefore Probability that you have to wait 5 minutes = 5/15 = 1/3

Therefore the probability that you have to wait more than 5 minutes = 1 - 1/3 = 2/3