The speed of the boat along the river is 12.3 km / h against the current of 6.9 km / h,

The speed of the boat along the river is 12.3 km / h against the current of 6.9 km / h, find the speed of the river, the proper speed of the boat.

Let us assume that the boat’s own speed is x km / h, and the river speed is y km / h.

The boat will go along the river at a speed:

x + y = 12.3 km / h.

And the boat will go against the current at a speed:

x – y = 6.9 km / h.

From the first equation we obtain that

x = 12.3 – y

and substitute this value into the second equation:

12.3 – y – y = 6.9,

2 * y = 12.3 – 6.9,

y = 5.4: 2,

y = 2.7 (km / h) – river flow speed.

x = 12.3 – 2.7 = 9.6 (km / h) – own speed of the boat.