Question

A jet ski with average speed of 60 mph left a pier by the river upstream. Fifteen minutes after its motor broke, and it began drifting downstream for 3 hours and 45 minutes until reached another pier, which was 2 miles away down the river from the original pier. What is the rate of the current, in mph?

Answer 1

4.25 mph

Explanation:

Speed, distance, and time are related by ...

... speed = distance/time

... distance = speed · time

___

If we let x represent the current speed in miles per hour, the number of miles the jet ski goes upstream is ...

... 0.25(60 -x)

The number of miles it drifts downstream is 2 more than that, or ...

... 0.25(60 -x) +2

The speed it drifts downstream is x, and that speed is equal to the distance divided by the time (3.75 hours):

... x = (0.25(60 -x) +2)/3.75

Multiplying by 3.75, we have ...

... 3.75x = 17 - 0.25x

... 4x = 17

... x = 17/4 = 4.25

The rate of the current is 4.25 miles per hour.