Two motor ships simultaneously departed from one pier in opposite directions. The speed of the first ship is 21 km / h

Two motor ships simultaneously departed from one pier in opposite directions. The speed of the first ship is 21 km / h, which is 7/9 of the speed of the other ship. How long will the distance between the ships be 144 km?

Let the speed of the second ship be x km / h. We know that 7: 9 of this speed is equal to the speed of the first motor ship – 21 km / h.
x * 7: 9 = 21
x * 7 = 21 * 9
x = 3 * 9 = 27 (km / h) – speed of the second motor ship.
Let the distance between them be 144 km in hours.
21 * y + 27 * y = 144
48 * x = 144
x = 3 (h) – the time after which the distance between the ships will be 144 km.
Answer: in 3 hours the distance between the ships will be 144 km.