Question

3. A crew team rows a boat at a rate of 20 km/h in still water. Beginning at time = 0 minutes, the team rows for 30 minutes up a river (against the current). The speed of the current is 1.5 km/h. How long will it take the crew team to row down the river (with the current) to return to the starting point?
30 min
22 min
26 min
24 min

Answer 1

The right answer for the question that is being asked and shown above is that: "26 min." A crew team rows a boat at a rate of 20 km/h in still water. Beginning at time = 0 minutes, the team rows for 30 minutes up a river (against the current). The speed of the current is 1.5 km/h. It will take 26 minutes the crew team to row down the river.

Answer 2

Going upstream against the current gives a net speed equivalent to the speed at still water minus the speed of the current. Consequently, the speed downstream gives a net speed equal to the speed at still water plus the speed of the current, making it travel faster. The solution is:

UPSTREAM

v = 20 - 1.5 = 18.5 km/h
t = 30 mins or 0.5 hours
distance = 18.5km/h (0.5 h) = 9.25 km

DOWNSTREAM
for the same distance of 9.25 km:

v = 20 + 1.5 = 21.5 km/h
t = 9.25km / 21.5 km/h = 0.43 hours or 25.8 mins  =  26 mins --> FINAL ANS.