Discussion

A tap having diameter 'd' can empty a tank in 84 min. How long another tap having diameter '2d' take to empty the same tank?

Answer: A

Flow through a pipe is proportional to its cross-sectional area, which varies as the square of the diameter.

\[ \frac{(2d)^{2}}{d^{2}} = 4 \]

Four times the flow means a quarter of the time:

\[ \frac{84}{4} = 21 \text{ minutes} \]

Halving the time to 42 minutes would assume flow proportional to diameter rather than to area.

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