Answer:
Option A is correct, i.e. 3 mph.
Step-by-step 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.
