A man launches his boat from point A on a
bank of a straight river, 2 km wide, and wants to reach
point B, 2 km downstream on the opposite
bank, as quickly as possible (see the figure below). He could row
his boat directly across the river to
point C and then run to B, or he
could row directly to B, or he could row to some
point D between C and B and
then run to B. If he can row 6 km/h and run 8 km/h,
where should he land to reach B as soon as
possible? (We assume that the speed of the water is negligible
compared to the speed at which the man rows.)