The motor boat set off along the river from one pier to another, and after 2.5 hours it returned back

| June 20, 2021 | education

The motor boat set off along the river from one pier to another, and after 2.5 hours it returned back, spending 15 minutes on anchorage. Find the speed of the river if the boat’s own speed is 18 km / h and the distance between the marinas is 20 km.

Let’s make an equation in which we write down the speed of the river as x km / h.

In this case, the speed of the boat against the current will be equal to: 18 km / h.

The time it took the boat to travel against the current will be: 20 / (18 – x).

The time spent on the way downstream will be equal to: 20 / (18 + x).

We convert the parking time from 15 minutes to a decimal fraction.

We get:

15/60 = 1/4 = 0.25.

Let’s write the equation:

20 / (18 + x) + 0.25 + 20 / (18 – x) = 2.5 hours.

Let’s get rid of denominators.

20 * (18 – x) + 20 * (18 + x) = 2.25 * (18 – x) * (18 + x).

72 + 72 = 145.8 – 0.45 * x ^ 2.

0.45 * x ^ 2 = 1.8.

x ^ 2 = 1.8 / 0.45 = 4.

x = 2 km / h.

Answer: The speed of the river is 2 km / h.



One of the components of a person's success in our time is receiving modern high-quality education, mastering the knowledge, skills and abilities necessary for life in society. A person today needs to study almost all his life, mastering everything new and new, acquiring the necessary professional qualities.