The speed of a boat in still water is v. The boat is to make a round

Question:

The speed of a boat in still water is v. The boat is to make a round trip in a river whose current travels at speed u. Derive a formula for the time needed to make a round trip of total distance D if the boat makes the round trip by moving
(a) Upstream and back downstream,
(b) Directly across the river and back, we must assume u