Posted by Minte on Thursday, December 27, 2012 at 10:48pm.
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.
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