The time X it takes for Michael to drive to school varies from day to day depending on weather and traffic conditions. Michael wishes to estimate the mean speed at which he drives.
To this end, he estimates the mean time E{X} computing the average time for a significant number of trips, and divides the distance to school by the estimate obtained. Another way is to compute the average speed in each trip (that is, to divide the distance by the time for each trip), and after that to compute the average speed for a number of trips. Which way is more precise? Is the mean speed over or underestimated when the worst way is used?