two taps A and B can fill a tank in 5 and 7 hours respectively .how many hours after opening tab A should tab B be opened , so that the tank gets filed 2 hours after opening tab B?

two taps A and B can fill a tank in 5 and 7 hours respectively .how many hours after opening tab A should tab B be opened , so that the tank gets filed 2 hours after opening tab B?