The motor boat covered 4 km upstream and 1 km upstream in 20 minutes, find the boat’s own speed if the current speed is 2 km / h.

X km / h – boat speed
х + 2 km / h – downstream speed
x-2 km / h – against the stream
20 minutes = 1/3 hours
4 / (x + 2) + 1 / (x-2) = 1/3
4 * 3 (x-2) +3 (x + 2) = (x + 2) (x-2)
12x-24 + 3x + 6 = x²-4
x² -15x + 14 = 0
D = 15²-4 * 14 = 169 = + – 13²
x1 = (15-13) / 2 = 1 – does not fit the solution
x2 = (15 + 13) / 2 = 14 (km / h) – boat speed