A cruise boat travels 36 miles downstream in 3 hours and returns to its starting point upstream in 6 hours. Find the speed of the stream.
A) $9 \mathrm{mph}$
B) $21 \mathrm{mph}$
C) $12 \mathrm{mph}$
D) $3 \mathrm{mph}$

Question

Quizplus · Accepted Answer

The speed of the stream can be found by calculating the difference between the boat's speed in still water and its speed relative to the ground, then dividing by 2. The boat's speed downstream is $36 \, \text{miles} / 3 \, \text{hours} = 12 \, \text{mph}$, and upstream is $36 \, \text{miles} / 6 \, \text{hours} = 6 \, \text{mph}$. The boat's speed in still water would be the average of these two speeds, $ (12 + 6) / 2 = 9 \, \text{mph}$. The speed of the stream is the difference between the boat's speed in still water and its speed upstream or downstream, so $12 - 9 = 3 \, \text{mph}$ or $9 - 6 = 3 \, \text{mph}$.

A Cruise Boat Travels 36 Miles Downstream in 3 Hours 9mph9 \mathrm{mph}9mph

A Cruise Boat Travels 36 Miles Downstream in 3 Hours $9 \mathrm{mph}$