Question 741712: 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?
Found 2 solutions by lynnlo, ikleyn:
Answer by lynnlo(4176)   (Show Source):
Answer by ikleyn(53763)  (Show Source):
You can put this solution on YOUR website! .
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?
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First, they row against the current with the speed (20-1.5) = 18.5 km/h for 0.5 of an hour.
During this time, they covered the distance 0.5*18.5 kilometers up the river.
Then they row with the current with the speed (20+1.5) = 21.5 km/h for 0.5 of an hour.
During this time, they covered the distance 0.5*21.5 kilometers down the river.
"How much farther did they travel" is the difference
0.5*21.5 - 0.5*18.5 = 0.5*(21.5 - 18.5) = 0.5*3 = 1.5 kilometers.
ANSWER. They traveled 1.5 kilometers farther.
Notice this remarkable fact: in (30 + 30) minutes, or 1 hour, they "traveled farther"
exactly the distance which the current "travels" in one hour.
It is so, because the current moved them with its speed of 1.5 km/h during all the time of their travel.
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