The boat, whose own speed is 8 km / h, traveled along the river a distance equal to 15 km downstream and the same distance upstream. Find the speed of the river if the time taken for the entire journey is 4 hours.
To solve the problem, we compose an equation in which the speed of the river flow will be equal to the number x.
In this case, the time that the boat spent on the way along the river against the current will be equal.
15 / (8-x).
And the time that the boat spent on the way upstream, respectively.
15 / (8 + x).
In this case, the sum of time on the way behind and against the current will be equal to the total time, namely 4 hours.
15 / (8-x) + 15 / (8 + x) = 4 hours.
120 + 15 * x + 120-15 * x = 256-4 * x²
240 = 256-4 * x²
4 * x² = 256-240
4 * x² = 16
x² = 16/4
x² = 4
x₁ = 2.
x₂ = -2. Since x <0, we choose 2 km / h as an answer.
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