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Six equal pipes fill a swimming pool in 20 hours. If only four of them are used, how long will the pool take to fill?

Competition practicePipes & Cisterns

A24 h
B18 h
C30 h
D $13 \frac{1}{3} h$

Step-by-Step Solution

Total work

= 6 \times 20 = 120

pipe-hours. With 4 pipes:

\frac{120}{4} = 30

h

. (13 \frac{1}{3}

h wrongly assumes fewer pipes finish sooner.)

Answer: C) 30 h

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