задача по математика

[Chacho]

40min=(2/3)h
15min=(1/4)h
Vпо теч = V+3 km/h

(V+3).(2/3) + V.(1/4) = 13
2V/3 + 2 + V/4 = 13
8V/12 + 3V/12 = 11
11V = 12.11
V=12km/h