The boat sailed 40 km along the river and 36 km along the lake in 4 hours. Find the boat’s own speed if the river flow is 2 km / h.

Let x be the speed of the boat, then
х + 2 – speed of the boat along the river,
40 / (x + 2) – the time during which the boat passed along the river,
36 / x – the time during which the boat sailed on the lake.
Based on the data in the problem statement, we compose an equation and determine what the boat’s own speed is equal to:
40 / (x + 2) + 36 / x = 4
Let us bring the fractions to a common denominator.
(40 x + 36x + 72) / x2 + 2x = 4
4×2 + 8x = 40x + 36x +72
4×2 – 68x -72 = 0
x2 – 17x – 18 = 0
D = 289 + 72 = 361
√D = 19
x1 = (17 + 19) / 2 = 18
x2 = (17 – 19) / 2 = -1
The speed of the boat cannot be 1 km / h, so its speed is 18 km / h.
Answer: the speed of the boat is 18 km / h.