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A tank is one-fourth full. Two pipes that can fill it in 15 minutes and 20 minutes are opened together. How long will they take to fill the rest?

Competition practicePipes & Cisterns

A $8 \frac{4}{7}$ min
B7.5 min
C $5 \frac{1}{7}$ min
D $6 \frac{3}{7}$ min

Step-by-Step Solution

Together:

\frac{1}{15} + \frac{1}{20} = \frac{7}{60}

per min. Remaining

\frac{3}{4} : \frac{3}{4} \times \frac{60}{7} = \frac{45}{7} = 6 \frac{3}{7}

min

. (8 \frac{4}{7}

min is the time for a full tank.)

Answer: D)

6 \frac{3}{7}

min

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