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 10 hours. If it is desired that the tank should be filled, from empty, exactly 8 hours after the inlet pipe is turned on at 9:30am, then at what time should the outlet pipe be opened ?  Choose the correct option.
A) 11:30am
B) 10:30am
C) 9:30am
D) 8:30am
E) 7:30am
In 8 hours the inlet pipe will pump in water equal to 8*1/5 = 8/5 of the pool. So, 3/5 more than necessary, which should be compensated by the outlet pipe. To pump out 3/5 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 + 8 hours - 6 hours= 11:30am.   11:30amnswer: 11:30am
<result>A</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 10 hours. If it is desired that the tank should be filled, from empty, exactly 8 hours after the inlet pipe is turned on at 9:30am, then at what time should the outlet pipe be opened ?