A tank is fitted with 4 large and 10 small filling pipes. Through each large pipe, water flows twice as rapidly as through each small pipe. All the pipes were opened simultaneously when the tank was empty. When the tank was expected to be full, it was found to be only two-thirds full, due to a leak at the bottom. The leak was then plugged immediately but only by 50%. If all the pipes can together fill up the tank (if there is no leak) in 7 1/2 hrs, how much more time is needed for the tank to be full, after the leak was partially plugged?

1) 3 1/2 hrs
2) 3 1/4 hrs
3) 2 3/4 hrs
4) 2 1/2 hrs
5) 3 hrs