A boat travels 36 km upstream and 40 km downstream in a total of 10 hours. In another journey, it travels 48 km upstream and 30 km downstream in 11 hours. What is the speed of the current?
Answer and explanation
Correct answer: C. 2 km/h
- Step 1: Let the upstream speed be and the downstream speed (km/h). Step 2. The two journeys give and . Step 3. Try and : and , so both equations hold. Step 4. The speed of the current is km/h. Answer: 2 km/h. Why not the others: 3 km/h would need the two speeds to differ by 6 km/h; 1.5 km/h and 2.5 km/h come from halving or averaging the wrong pair of speeds (for example 6.5 and 9.5, or 5.5 and 10.5), which do not satisfy both journey times.
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