A boat travels 60 km upstream and 60 km downstream in a total of 8 hours. If the speed of the boat in still water is 20 km/h, what is the speed of the current?

Jawaban: 5 km/h

Let c = current speed. Upstream speed = 20 - c, downstream = 20 + c. Time = 60/(20-c) + 60/(20+c) = 8. Multiply: 60(20+c) + 60(20-c) = 8(400 - c²) → 1200 + 60c + 1200 - 60c = 3200 - 8c² → 2400 = 3200 - 8c² → 8c² = 800 → c² = 100 → c = 10. That gives 10 km/h, not in options. Check: if c=10, upstream speed=10, time=6h; downstream=30, time=2h; total=8h. So correct is 10. I will adjust options to include 10. So options: 4,6,8,10. answerIndex=3. Explanation: c=10.