Question

A family travels 18 miles downriver and returns. It takes 8 hours to make the round trip. Their rate in still water is twice the rate of the current. Find the rate of the current.

Answer 1

3 mph

Explanation:

You want to know the rate of the current if the boat speed is twice the current speed and it takes 8 hours for a trip 18 miles downriver and back.

Time

The relationship between time, speed, and distance is ...

time = distance/speed

If c represents the rate of the current, then the total trip time is ...

18/(2c +c) +18/(2c -c) = 8

6/c +18/c = 8

24/8 = c . . . . . . . . . combine terms, multiply by c/8

c = 3 . . . . . . the speed of the current is 3 miles per hour

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