Moving along the river, the distance of 72 km is covered by the motor boat in 4 hours, and the raft in 24 hours.

Moving along the river, the distance of 72 km is covered by the motor boat in 4 hours, and the raft in 24 hours. Calculate the speed of the motor boat when moving against the river.

Let x be the speed of the river, y the speed of the boat
boat 72 y + x 4 h – along the river
raft 72 x 24 h
Find x: x = 72/24, x = 3, then
y + 3 = 72/4
y + 3 = 18, y = 15
Then the speed of the boat against the flow of the river: y-x = 15-3 = 12
Answer: 12