Question

A boat going in the river downstream went 30km in 2 hours. The same boat going upstream reaches the same distance after 6 hours. The river flow speed and the boat's speed are constant throughout the whole journey. What is the river flow speed?

Answer 1

The river flow speed is 5km/h

Explanation:

Let's call the boat's speed B and the river's speed R. The movement downstream lets us know that the sum of the speeds gets us 30km in 2 hours or a speed of 15 km per hour (30/2), so:

`B + R = 15`

The movement upstream tells us that the boat's speed minus the river's speed lets the boat do the same distance fo 30 km in 6 hours so a total speed of 5 km per hour (30/6), which gives us:

`B - R = 5`

Armed with these two equations we can sum each of the terms so we get:

`(B+B)+(R-R)=15+5`

`2B=20`

`B=10`

So the boat's speed is 10km/h and when you plug 10 into any of the two equations you will get that the river's speed is 5km/h.