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A tap can fill a tub in 24 hours. Due to a leak at the bottom of the tub, the tap fills the tub in 36 hours. If the tub is full, how much time will the leak take to empty it?

  • A.72 hours
  • B.48 hours
  • C.60 hours
  • D.96 hours

Answer: A

Work in rates: a pipe that fills a tank in \( n \) units of time fills \( \tfrac{1}{n} \) of it per unit, and rates add (an emptying pipe counting negative).

\[ \frac{1}{24} - \frac{1}{L} = \frac{1}{36} \]

\[ \frac{1}{L} = \frac{3 - 2}{72} = \frac{1}{72} \quad\Longrightarrow\quad L = 72 \text{ hours} \]

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