Discussion
Pipes A and B can fill an empty tank in 15 hours and 18 hours, respectively. Pipe C is an emptying pipe. If all the three pipes are opened for 5 hours, then \(\frac{1}{9}\) th part of the tank can be filled. Pipe A and C together can empty a full tank in
Answer: C
Pipes A and C together can empty a full tank in 30 hours. In one hour, A fills 1/15 and B fills 1/18, which together make 11/90 of the tank. With C also open, 1/9 of the tank is filled in 5 hours, which is 1/45 per hour.
\[ \frac{11}{90} - c = \frac{1}{45} \]
\[ c = \frac{9}{90} = \frac{1}{10} \]
So C empties the tank at 1/10 per hour. A and C together then change the tank by 1/15 - 1/10 = -1/30 of it per hour, so they empty a full tank in 30 hours. The options 25, 28 and 35 hours do not follow from these rates.
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