The speed of a boat in still water is 25 mph. The boat is traveling on a river with a current flowing south at 10 mph. At what angle upstream should the boat head to travel due west?

To travel due west, the boat’s northward velocity must cancel the river’s 10 mph southward current. If the boat heads at an angle θ upstream from west, its northward speed is 25 sin θ, so 25 sin θ = 10. Thus θ = sin⁻¹(10/25) ≈ 23.58°, the angle in choice A.

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