The speed of a motor boat is that of the current of water as 36:5. The boat goes along with the current in 5 hours 10 min, it will come back in_______?
Correct answer: C. 6 hours 50 min
- A. 5 hours 50 min
- B. 6 hours
- C. 6 hours 50 min
- D. 12 hours 10 min
Explanation
Take the boat's still-water speed and current speed as 36 units and 5 units, so downstream and upstream speeds are 41 units and 31 units. The return time is 5 hours 10 minutes x 41/31 = 310 minutes x 41/31 = 410 minutes = 6 hours 50 minutes. The likely mistake is option a, 5 hours 50 minutes, which comes from treating the upstream speed as only slightly slower instead of using the ratio 31:41.
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