posted by joe .
The speed of your ship in still water is 20 knots.Your ship traveled 70 miles downward in channel in the same amount of time it traveled 45 miles upward the channel.Calculate the rate of the channel current.
since time = distance/speed, and the times are equal,
Assuming nautical miles, so knots make sense,
70/(20+s) = 45/(20-s)
s = 100/23 = 4.35 knots