posted by lawrence .
A boat propelled so as to travel with a speed of 0.50m/s in still water, moves directly (in a straight line) across a river that is 60m wide. The river flows with a speed of 0.30m/s. How long in seconds does it take the boat to cross the river?
Presumably the boat is aimed perpendicular to the parallel banks, but the current carries it downstream.
Just for orientation, let's say the river flows east, and the boat crosses to the north.
In that case, we can draw a diagram involving the relative velocities. The boat's speed is .5 at 90 deg.
The river's speed is .3 at 0 deg
So, the boat's resultant speed is sqrt(.5^2 + .3^2) = .583 at 59.1 deg
At that direction, the distance travelled is 60 sec 30.9 deg = 70m
70m/.583s = 2 minutes