One pipe can fill a pool twice as faster as the second pipe. When both pipes are opened, they fill the pool in five hours. How long would it take to fill the pool if only the slower pipe is used?  Choose the correct answer:
A) 10  B) 15  C) 15.5  D) 12.5  E) 18
Say the rate of the slower pipe is R pool/hour, then the rate of the faster pipe would be 2R. Since when both pipes are opened, they fill the pool in five hours, then their combined rate is 1/5 pool/hour.   Thus we have that R + 2R = 1/5 --&gt; R = 1/15 pool/hour --&gt; time is reciprocal of rate thus it's 15/1= 
<gadget id="calculator">15/1</gadget>
<output>15</output>
15 hours.   Answer: 15.
<result>B</result>
One pipe can fill a pool twice as faster as the second pipe. When both pipes are opened, they fill the pool in five hours. How long would it take to fill the pool if only the slower pipe is used?