A bathtub can be filled by a cold pipe in 15 minutes and by a hot pipe in 10 minutes. A man opens both the taps at 7 a.m. and leaves the bathroom. He returns at the time when the tub was expected to be full, but he observes that the waste pipe in the bottom was open and the tub was not full. He now closes it, and it takes 4 more minutes before the tank is full. After using the bathtub for 10 minutes, he opens the waste pipe. At what time will the tub be empty?