A boat goes downstream for 30 km, and against the current it floats for 20 km

A boat goes downstream for 30 km, and against the current it floats for 20 km and spends the same amount of time. How long will the boat sail for the same amount of time?

Let’s designate: x hours – the time that the boat spends 30 km upstream or 20 km upstream.
The downstream speed will be 30 / x km / h, the upstream speed will be 20 / x km / h.
The difference between downstream speed and upstream speed will give twice the current speed. Let’s find the speed of the current:
(30 / x – 20 / x) / 2 = (10 / x) / 2 = 5 / x km / h;

Let’s find the distance that the boat will travel in x hours:
(5 / x) * x = 5 km;

Answer: during the same time the boat will cover 5 km.