The boat traveled along the river the distance from the city to the village at a speed of 20 km / h, and back, against the current

The boat traveled along the river the distance from the city to the village at a speed of 20 km / h, and back, against the current, at a speed of 15 km / h. What is the average speed of the boat along the entire route?

Suppose the distance between city and town is 1.

In this case, the conditional time that the boat spent on the way downstream will be equal to:

1/20 = 1/20.

The return trip time was:

1/15 = 1/15.

We find the total conditional time during which the boat was on the way.

Let’s summarize the found parts.

1/15 + 1/20 = 4/60 + 3/60 = 7/60.

The total conditional distance that the boat has traveled will be:

1 + 1 = 2.

So its average speed will be equal to:

2/7/60 = 2 * 60/7 = 120/7 = 17 1/7 km / h.

Answer: 17 1/7 km / h.