A bus scheduler studied the time taken to travel a particular route during off-peak times over a period of several months. He found that the time taken for the outbound journey had a mean of 28.5 minutes and a standard deviation of 3.2 minutes, while the time taken for the inbound journey had a mean of 30.1 minutes and a standard deviation of 2.6 minutes. The scheduler is interested (among other matters) in the difference between the times taken to complete the outbound and inbound legs of a round trip. Define D such that it is “outbound trip” ― “inbound trip”. Determine the standard deviation of the variable D.