The boat with a speed of 15 km / h sailed 2 hours along the river and 3 hours against the current.

The boat with a speed of 15 km / h sailed 2 hours along the river and 3 hours against the current. What is the distance the boat swam this time if the speed of the current is 2 km / h.

Let’s designate the own speed of the boat as vk, and the speed of the current as vt. Then the boat sailed for 2 hours at a speed:

vdriver = vk + vt = 15 + 2 = 17 km / h.

And he swam a distance equal to:

s = t * v downstream = t * (vk + vt) = 2 * 17 = 34 km.

Then the boat sailed for 3 hours at a speed:

v upstream = vk – vt = 15 – 2 = 13 km / h.

And he swam a distance equal to:

s = t * v upstream = t * (vk – vt) = 3 * 13 = 39 km.

The boat sailed in total:

34 + 39 = 73 km.

Answer: the boat sailed 73 km.