The boat sailed 72 km downstream in 2 hours, and upstream in 3 hours. How many hours the rafts will sail this distance.

The speed of the rafts is equal to the speed of the river, we take it as x,
y is the speed of the boat, then we get two equations:
72 / (y + x) = 2 and 72 / (y-x) = 3
Express y from the first
(y + x) = 72/2 = 36
y = 36-x
and substitute in the second:
(36-x-x) = 72/3
36-2x = 24
2 * x = 36-24
x = 12/2 = 6 km / h
Then the desired answer is:
72/6 = 12 hours.