One pipe can fill a pool 1.25 times faster than a second pipe. When both pipes are opened, they fill the pool in 4 hours. How long would it take to fill the pool if only the slower pipe is used?	Choose the correct choice.
A) 11.25 B) 11.52 C) 1.25 D) 9 E) 7.2
Say the rate of the slower pipe is R pool/hour, then the rate of the faster pipe would be 1.25R=5R/4. Since when both pipes are opened, they fill the pool in four hours, then their combined rate is 1/4 pool/hour.   Thus we have that R + 5R/4 = 1/4 --&gt; R = 1/9 pool/hour --&gt; time is reciprocal of rate thus it's 9/1= 
<gadget id="calculator">9/1</gadget>
<output>9</output>
9 hours.   Answer: 9.
<result>D</result>
One pipe can fill a pool 1.25 times faster than a second pipe. When both pipes are opened, they fill the pool in 4 hours. How long would it take to fill the pool if only the slower pipe is used?