posted by Minte .
A boat takes 4 min. to cross a river 90m wide directly when there is no current & 5min. When there is current. find the velocity of the current.
With no river flow, the boat crosses the river at 90/4 = 22.5 meters/min.
With river flow, it's speed relative to water is the same as above, but it crosses at a speed of 90/5 = 18 m/min, relative to and perpendicular to the shore.
Consider the vector right triangle with sides of river speed, speed relative to shore, and speed relative to water.
If v is the river speed,
v^2 + 18^2 = 22.5^2
Solve for v.