A boat is able to move through still water at 20 m/s.It makes a round trip to a town 3.0 km upstream.If the river flows at 5 m/s, the time required for this round trip is:
A)120 s
B)150 s
C)200 s
D)300 s
E)320 s

Question

Quizplus · Accepted Answer

The speed of the boat in still water is 20 m/s, and the speed of the river is 5 m/s. When going upstream, the effective speed of the boat is $20 - 5 = 15$ m/s, and when going downstream, it is $20 + 5 = 25$ m/s. The distance to the town is 3.0 km, which is 3000 m. The time to go upstream is $3000 m / 15 m/s = 200$ s, and the time to go downstream is $3000 m / 25 m/s = 120$ s. The total time for the round trip is $200 s + 120 s = 320$ s.

A Boat Is Able to Move Through Still Water at 20