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Taps A, B and C fill a tank in 10, 20 and 30 minutes respectively. If all three are opened together, how long does the empty tank take to fill?

Competition practicePipes & Cisterns

A60 min
B $5 \frac{5}{11}$ min
C20 min
D6 min

Step-by-Step Solution

\frac{1}{10} + \frac{1}{20} + \frac{1}{30} = \frac{6 + 3 + 2}{60} = \frac{11}{60}

per minute →

\frac{60}{11} = 5 \frac{5}{11}

min. (20 min is the average time; 60 min the sum.)

Answer: B)

5 \frac{5}{11}

min

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