A man launches his boat from point A on a bank of a straight river, 3 km wide, and wants to reach point B, 1 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, how far (in km) downstream from C 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.

A C D B