Discussion
Two pipes can fill a tank in 10 and 12 hours respectively while a third pipe can empty the full tank in 20 hours. If all three pipes operate simultaneously, in how much time will the tank be filled?
Answer: B
Work in rates: someone who finishes a job in \( n \) days does \( \tfrac{1}{n} \) of it a day, and rates add.
\[ \frac{1}{10} + \frac{1}{12} - \frac{1}{20} = \frac{6 + 5 - 3}{60} = \frac{8}{60} = \frac{2}{15} \]
\[ t = \frac{15}{2} = 7.5 \text{ h} = 7 \text{ hr } 30 \text{ min} \]
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