According to The Earth: Structure, Composition and Evolution (The Open University, S237), for earthquakes with a magnitude of 7.5 or greater on the Richter scale, the time between successive earthquakes has a mean of 437 days and a standard deviation of 399 days. Suppose that you observe a sample of four times between successive earthquakes that have a magnitute 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.)