Answer

The overall mean travel time for bus and taxi passengers is 36 minutes on Sunday and 32 minutes on Monday. The overall mean travel time is correlated with the day of the week--it is four minutes lower on Monday than on Sunday. However, this correlation is spurious, since the mean travel time by bus is lower on Sunday than on Monday, and so is the mean travel time by taxi. The reason that the overall mean travel time is lower on Monday is that a larger proportion of travellers use taxis on Monday, and taxis are faster than buses.