When B is turned on A has put 3.6(2) = 7.2 litres of water into the tank So there remain 55.2-7.2 = 48 liters to fill.So we have:
3.6T + 2.4T = 48where T is the time it takes to fill the rest of the tank...Simpifying we have
6T = 48
T = 8 minutes
So, since B was on for 8 minutes it put 2.4(8) = 19.2 liters into the tank.