4. Alan runs on a treadmill each day for as long as he can at 7 miles per hour. The length of time for which he runs is normally distributed with a mean of 21.6 minutes and a standard deviation of 1.8 minutes.

1. Calculate the probability that on any one day Alan will run for less than 20 minutes.
2. Estimate the number of times in a ninety-day period that Alan will run for more than 24 minutes.
3. On a particular day Alan is still running after 22 minutes. Find the probability that he will stop running in the next 2 minutes.