posted by james .
A ferryboat traveling at a speed of 30 km/h attempts to cross a river with a current of 5 km/h. What is the boat's speed relative to the shore?
It depends upon whether the ferry is travelling straight across (but aimed upstream) or aimed straight across (but drifting downsteam). The question does not make clear which it is.
In the first case, the answer is sqrt(30^2 -5^2) = 29.6 mph
In the secnd case, the answer is sqrt(30^2 + 5^2) = 30.4 mph