A boat departed from one bank of the river to the opposite one at a speed of 60 km / h, and 7 minutes

A boat departed from one bank of the river to the opposite one at a speed of 60 km / h, and 7 minutes later a second boat departed in the same direction at a speed of 80 km / h. What is the length of the boats’ path (river width) if the second boat approached the opposite bank of the river 8 minutes earlier than the first boat?

According to the condition of the problem, the second boat set out 7 minutes earlier and came to the opposite bank 8 minutes earlier, that is, it was on the way to

7 + 8 = 15 minutes or 1/4 hour less.

Let the width of the river be x km, then, according to the condition of the problem, the following equation can be drawn up:

x / 60 – x / 80 = 1/4,

4 * x / 240 – 3 * x / 240 = 1/4,

(4 * x – 3 * x) / 240 = 1/4,

x / 240 = 1/4,

x = 240/4,

x = 60 (km) – width of the river.

Answer: 60 km.