The motor ship sails the distance between the two cities along the river in 4 hours, and along the lake in 5 hours.

The motor ship sails the distance between the two cities along the river in 4 hours, and along the lake in 5 hours. how long does it take for a raft to cover the distance between these two cities?

1. Let’s designate: x km – distance between two cities.
2. Find the speed of the ship along the river. It is equal to x / 4 km / h.
3. Let’s find the speed of the ship on the lake, that is, the own speed of the ship. It is equal to x / 5 km / h.
4. Let’s find the speed of the river, which is found as the difference between the speed of the ship along the river and its own speed:
x / 4 – x / 5 = 5x / 20 – 4x / 20 = x / 20 km / h.
5. The raft can only sail at the speed of the river. He will sail the distance between the two cities in time:
x / (x / 20) = 20x / x = 20 hours.
Answer: The raft will sail the distance between the two cities in 20 hours.