The boat sailed 72 km between the piers along the river in 2 hours and against the current in 3 hours. How many hours do rafts take this distance?
Once the boat sailed 72 km in 2 hours, then along the river it moved at a speed:
1) 72/2 = 36 (km / h).
Against the current, the boat sailed 72 km in 3 hours, which means that it was moving against the river at a speed:
2) 72/3 = 24 (km / h).
3) (36 + 24) / 2 = 30 (km / h) – own speed of the boat.
4) 36 – 30 = 6 (km / h) – current speed.
The rafts move at the speed of the current.
5) 72/6 = 12 (h) – the distance of 72 km will be covered by rafts.
Answer: 12 hours.
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