A dual sink has a right side and left side. Working alone, the right side can fill up in 5 hours. Working alone, the left side can empty in 15 hours. If it is desired that the sink should be filled, from empty, exactly 6 hours after the right side is turned on at 9:30am, then at what time should the left side also be turned on? Choose one:
A) 10:00am  B) 10:45am  C) 12:00pm  D) 12:30pm  E) 1:30pm
In 6 hours the right side will fill up with water equal to 6*1/5 = 6/5 of the sink. So, 1/5 more than necessary, which should be compensated by the left side of the sink. To pump out 1/5 of the water the left side needs  15/5= 
<gadget id="calculator"> 15/5</gadget>
<output>3</output>
3 hours.   Therefore, the left side of the sink should be turned on at 9:30am + 6 hours - 3 hours= 12:30pm.   Answer: 12:30pm.
<result>D</result>
A dual sink has a right side and left side. Working alone, the right side can fill up in 5 hours. Working alone, the left side can empty in 15 hours. If it is desired that the sink should be filled, from empty, exactly 6 hours after the right side is turned on at 9:30am, then at what time should the left side also be turned on?