For earthquakes with a magnitude 7.5 or greater on the Richter scale, the time between successive earthquakes has a mean of

456 days and a standard deviation of 379 days. Suppose that you observe a sample of four times between successive earthquakes that have a magnitude of 7.5 or greater on the Richter scale.

a. On average, what would you expect to be the mean of the four times?

b. How much variation would you expect from your answer in part (a)? (Hint: Use the three-standard-deviations rule.)