The raft departed from the coast and sailed along the river. 25 minutes later a motor boat followed it with its own

| April 11, 2021 | education

The raft departed from the coast and sailed along the river. 25 minutes later a motor boat followed it with its own speed equal to 15 km / h and swam 1.5 km and caught up with the raft. Find the speed of the river current.

Let the river flow speed be x km / h. Then in 25 minutes the raft sailed 25/60 * x = 5/12 * x km. The speed of a motor boat along the river is equal to the sum of the boat’s own speed and the speed of the current: 15 + x km / h.
Let’s determine how long it took the boat to sail 1.5 km:
1.5 / (15 + x) hours.
During the same time, the raft sailed 1.5 – 5/12 * km.
Let’s compose an equation and find its root:
1.5 * x / (15 + x) = 1.5 – 5 * x / 12.
The root of this equation: x = 3. Hence, the speed of the river in this problem is 3 km / h.



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