Discussion
Pipe P can fill a tank in 4 hours, pipe Q in 8 hours and pipe R in 24 hours. If all the pipes are open, in how many hours will the tank be filled?
Answer: B
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).
\[ \frac{1}{4} + \frac{1}{8} + \frac{1}{24} = \frac{6 + 3 + 1}{24} = \frac{10}{24} = \frac{5}{12} \]
\[ t = \frac{12}{5} = 2.4 \text{ hours} \]
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