Question

A crew team rows a boat at a rate of 20 km/h in still water. In practice on a river, the team rows for 30 minutes up the river (against the current), and then for 30 minutes down the river (with the current). The speed of the river current is 1.5 km/h. How much farther did they travel in the second 30 minutes?

2.0 km

3.0 km

1.5 km

0.75 km

Answer 1

half an hour up the river, 0.75 km less than still water
half an hour down the river, 0.75 more than in still water
so
0.75 + 0.75 = 1.5 km

Answer 2

d1 = V*T = (20-1.5) * 30/60 = 9.25 km up stream.

d2 = V*T = (20+1.5) * 30/60 = 10.75 km down stream.

d2-d1 = 10.75 - 9.25 = 1.5 km farther.