A motorboat travels upstream and downstream in hours. It can also travel upstream and downstream in hours. What is the speed of the river current?

Answer and explanation

Correct answer: A. 5 km/h

(Equation 1)
(Equation 2).
  1. Step 1: Let be upstream speed and be downstream speed in . The two trip conditions give:
  2. Step 2: Multiply Equation 2 by : . Subtract this from Equation 1 to eliminate : . Substituting into Equation 2 gives .
  3. Step 3: Compute the speed of the river current: .
Answer
5 km/h.

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