A tank has both an inlet pipe and an outlet pipe. Working alone, the inlet pipe can fill up the tank in 5 hours. Working alone, the outlet pipe can empty out the tank in 15 hours. If it is desired that the tank should be filled, from empty, exactly 7 hours after the inlet pipe is turned on at 9:30am, then at what time should the outlet pipe be opened ?  Select:
A) 9:30am
B) 10:30am
C) 11:30am
D) 10:40am
E) 10:50am
In 7 hours the inlet pipe will pump in water equal to 7*1/5 = 7/5 of the pool. So, 2/5 more than necessary, which should be compensated by the outlet pipe. To pump out 2/5 of the water the outlet pipe needs 30/5= 
<gadget id="calculator">30/5</gadget>
<output>6</output>
6 hours.   Therefore, the outlet pipe should be opened at 9:30am + 7 hours - 6 hours= 10:30am.   Answer: 10:30am
<result>B</result>
A tank has both an inlet pipe and an outlet pipe. Working alone, the inlet pipe can fill up the tank in 5 hours. Working alone, the outlet pipe can empty out the tank in 15 hours. If it is desired that the tank should be filled, from empty, exactly 7 hours after the inlet pipe is turned on at 9:30am, then at what time should the outlet pipe be opened ?