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All four pipes in the figure are opened together on the empty tank. How long does it take to fill?

Competition practicePipes & Cisterns

A4 h
B24 h
C12 h
D2.4 h

Step-by-Step Solution

Per hour:

\frac{1}{6} + \frac{1}{12} - \frac{1}{8} - \frac{1}{24} = \frac{4 + 2 - 3 - 1}{24} = \frac{2}{24}

→ 12 h. (4 h ignores the outlets; 2.4 h counts them as inlets.)

Answer: C) 12 h

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