Question

A motorboat can go 8 miles downstream on a river in 20 minutes. It takes 30 minutes for the boat to go upstream the same 8 miles. Find the speed of the current. A 4 mph B 20 mph C 24 mph D 16 mph

Answer 1

A. 4 mph

Explanation:

For this multiple-choice question, the only reasonable answer is the correct one. The upstream speed is (8 mi)/(1/2 h) = 16 mph, so the speed of the current must be less than that.

The only answer choice less than 16 mph is 4 mph, the correct current speed.

_____

Downstream speed is (8 mi)/(1/3 h) = 24 mph. The speed of the current adds to the speed of the boat to give the downstream speed, and it subtracts from the speed of the boat to give the upstream speed. Then the current speed is half the difference of the travel speeds, (24 -16) mph/2 = 4 mph.