A boat and a motor boat must transport a group of people from the pier to the island and back.

| September 2, 2021 | education

A boat and a motor boat must transport a group of people from the pier to the island and back. The speed of the river they are going to walk is 3 km / h, the distance from the pier to the island is 21 km. The way to the island (upstream) takes twice as long for the boat as for the boat. How many minutes before the motor boat does the boat need to get off on the way back in order to come back at the same time as the boat, if it is known that the return trip of the boat takes 35 minutes?

There are 10 actions in solving this problem:
1). 21/35 = 0.6 (km / min) = 36 (km / h) – the speed of the motor boat on the way back (downstream).
2). 36-3 = 33 (km / h) – own speed of the motor boat.
3). 33-3 = 30 (km / h) – the speed of the motor boat from the pier to the island (upstream).
4). 21/30 = 0.7 (h) – travel time of a motor boat from the pier to the island (upstream).
5). 0.7 * 2 = 1.4 (h) – the time the boat travels from the pier to the island (upstream).
6). 21 / 1.4 = 15 (km / h) – speed of the boat from the pier to the island (upstream).
7). 15 + 3 = 18 (km / h) – own speed of the boat.
eight). 18 + 3 = 21 (km / h) – speed of the boat on the way back (downstream).
nine). 21/21 = 1 (h) = 60 (min) – the time of the boat’s return trip (downstream).
ten). 60-35 = 25 (min) – before the motor boat you need to go back to the boat.
Answer: 25 minutes before the motor boat you need to go back to the boat.



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