Question

A river is flowing at the rate of 2.4 miles an hour when a boy rows across it. If the boy rows at a still-water speed of 3.1 miles per hour and heads the boat perpendicular to the direction of the current. The ground speed of the boat is _____ miles per hour.

Answer 1

3.9 mi/h

Explanation:

If the boy is rowing perpendicular to the current, the two vectors form a right triangle.

AB represents the downstream current, BC is the speed across the river, and AC is the ground speed of the boat

AC^2 = 2.4^2 + 3.1^2 =5.76 + 9.61 = 15.37

AC = sqrt(15.37) = 3.9 mi/h

The boat's speed over the ground is 3.9 mi/h.