Question

Tinh can row at a rate of 6 mph in still water. It takes her 2 hours to row upstream from a dock to a park. She then rows back to the dock, and it only takes 40 minutes. What is the speed of the river current? 3 mph 12 mph 18 mph 48 mph

Answer 1

Option A (3)

Explanation:

Given that Tinh can row at a rate of 6 mph in still water.

Also it takes her 2 hours to row upstream from a dock to a park. She then rows back to the dock, and it only takes 40 minutes.

While upstream she travels against the stream but while downstream she travels along with the stream

Let x be the speed of boat in still water and y that of stream

Then since distance d is the same, time x speed =D

i.e. (x-y)120 = (x+y)40 after converting hours to minutes

80x=160y

x=2y

x=6 (given)

So y = 6/2=3 mph

Answer 2

Option A is correct, i.e. 3 mph.

Explanation:

Tinh can row at a rate of 6 mph in still water.

Suppose the speed of the river current is X mph.

Then Upstream speed = (6-X) mph.

And Downstream speed = (6+X) mph.

It takes her 2 hours to row upstream from a dock to a park. She then rows back to the dock, and it only takes 40 minutes.

It means Distance upstream = Distance downstream.

Speed upstream x Time upstream = Speed downstream x Time downstream.

(6-x) * 2 = (6+x) * 40/60

12 - 2x = 4 + 2x/3

12 - 4 = 2x/3 + 2x

8 = 2x/3 + 6x/3 = 8x/3

x = 3 mph.

Hence, option A is correct, i.e. 3 mph.