Sule's job is just a few bus stops away from his house. While it can be faster to take the bus to work than to walk, the travel time is mote variable due to traffic. The commute time B if Sule takes the bus to work on a randomly selected day has mean 12 minutes and standard deviation 4 minutes. The commute time W if Sule walks to work on a randomly selected day has mean 16 minutes and standard deviation 1 minute. Calculate and interpret the mean of the difference D=B-W in the time it would take Sule to get to work on a randomly selected day.