Discussion
A can do a piece of work in 4 hours; B and C together can do it in 3 hours, while A and C together can do it in 2 hours. How long will B alone take to do it?
Answer: C
Work in rates: someone who finishes a job in \( n \) days does \( \tfrac{1}{n} \) of it a day, and rates add.
\[ A = \frac{1}{4}, \qquad B + C = \frac{1}{3}, \qquad A + C = \frac{1}{2} \]
\[ C = \frac{1}{2} - \frac{1}{4} = \frac{1}{4} \]
\[ B = \frac{1}{3} - \frac{1}{4} = \frac{1}{12} \]
\[ t_{B} = 12 \text{ hours} \]
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