The boat sailed 72 km between the piers along the river in 2 hours and against the current

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.