A pipe 'N' can fill a tank in 1/4 hour and another pipe 'M' can fill it in 1/6 hour. Both pipes are opened to fill the empty tank, but after 1/15 hour a leak developed in the tank. The leak alone can empty the full tank in just 1/12 hour. If the both pipes were opened at 9 pm, then at what time will the tank be emptied?