Two taps 'A' and 'B' can fill a tank in 5 and 7 hours respectively. How many hours after opening tap A' should tap 'B' be opened, so that the tank gets filled 2 hours afteropening tap 'B'?

Two taps 'A' and 'B' can fill a tank in 5 and 7 hours respectively. How many hours after opening tap A' should tap 'B' be opened, so that the tank gets filled 2 hours afteropening tap 'B'?