A tank has both an inlet pipe and an outlet pipe. Working alone, the inlet pipe can fill up the tank in 4 hours. Working alone, the outlet pipe can empty out the tank in 4 hours. If it is desired that the tank should be filled, from empty, exactly 6 hours after the inlet pipe is turned on at 9:30am, then at what time should the outlet pipe be opened ?	Choices: A) 02:30 pm B) 03:30 pm C) 01:30 pm D) 04:30 pm E) 05:30 pm
In 6 hours the inlet pipe will pump in water equal to 6*1/4 = 6/4 of the pool. So, 2/4 more than necessary, which should be compensated by the outlet pipe. To pump out 2/4 of the water the outlet pipe needs 8/4= 
<gadget id="calculator">8/4</gadget>
<output>2</output>
2 hours.   Therefore, the outlet pipe should be opened at 9:30am + 6 hours - 2 hours= 01:30 pm.   Answer: 01:30 pm
<result>C</result>
A tank has both an inlet pipe and an outlet pipe. Working alone, the inlet pipe can fill up the tank in 4 hours. Working alone, the outlet pipe can empty out the tank in 4 hours. If it is desired that the tank should be filled, from empty, exactly 6 hours after the inlet pipe is turned on at 9:30am, then at what time should the outlet pipe be opened ?