The speed of the motor boat along the river is 15.6 km / h, upstream by 25% less. Find the speed of the boat on the lake.

1. Let X km / h be the own speed of the motor boat, or the speed of movement on the lake.

Y km / h – river flow speed.

Then (X + Y) km / h is the speed of the boat along the river.

(X – Y) km / h – boat speed against the current.

2. The boat speed upstream is 25% less than the downstream speed.

Let’s find the difference in kilometers.

25% is 25/100.

(25/100) * 15.6 = 3.9 km / h.

Thus, the speed of the boat against the river flow will be 15.6 – 3.9 = 11.7 km / h.

3. Received 2 equations with 2 unknowns.

X + Y = 15.6.

X – Y = 11.7.

Let’s add both equations.

2 * X = 15.6 + 11.7

2 * X = 27.3.

X = 27.3 / 2.

X = 13.65 km / h – own speed of the motor boat.

Answer: The speed of the boat on the lake is 13.65 kilometers per hour.