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The table shows how long each pipe takes on its own to fill the same tank. Which pair, opened together, fills the empty tank in exactly 8 hours?

Competition practicePipes & Cisterns

AP and R
BR and S
CQ and R
DP and Q

Step-by-Step Solution

P + Q : \frac{1}{12} + \frac{1}{24} = \frac{3}{24} = \frac{1}{8}

→ 8 h.

P + R

gives

\frac{8}{60}

(7.5 h);

R + S

gives

\frac{3}{40} (13 \frac{1}{3}

h);

Q + R

gives

\frac{11}{120}

.

Answer: D) P and Q

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