The boat sailed first 10 km along the river, and then 3 km along the lake in the absence of current.

The boat sailed first 10 km along the river, and then 3 km along the lake in the absence of current. find the speed of the river flow, if the boat’s own speed was constant and equal to 18 km / h, and the time spent by the boat for the entire journey was 40 minutes.

We will solve this problem by drawing up an equation;

Let x km / h – this is the value of the speed of the river flow;

Then 18 + x km / h is the value of the speed of this boat along the river;

10 / (18 + x) h – this is the time this boat spent on the way along the river;

3:18 = 1/6 h – this is the time this boat spent on the way along the lake;

We also know that this boat spent 40 minutes for the entire journey, therefore we compose an equation of the following form:

10 / (18 + x) + 1/6 = 2/3;

60 + 18 + x = 4 * (18 + x);

78 + x = 72 + 4 * x;

3 * x = 6;

x = 6: 3;

x = 2.

2 km / h is such a value for the speed of the river.