pipes P and Q would fill a cistern 18 and 24 minutes respectively. Both pipes being opened, find when the first pipe must be turned off so that the cistern may be just filled in 12 minutes?  Choose the correct choice.
A) 2	B) 9	C) 8	D) 7	E) 5
x/18 + 12/24 = 1   x = 9   Answer: 9
<result>B</result>
pipes P and Q would fill a cistern 18 and 24 minutes respectively. Both pipes being opened, find when the first pipe must be turned off so that the cistern may be just filled in 12 minutes?