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A and B together finish a job in 10 days. If A's efficiency were doubled and B's halved, they would finish in 8 days. How long does A take alone?

Competition practiceTime & Work / Pipes

A15 days
B30 days
C12 days
D20 days

Step-by-Step Solution

a + b = \frac{1}{10}

and

2 a + \frac{b}{2} = \frac{1}{8}

. Substituting

b = \frac{1}{10} - a : \frac{3 a}{2} = \frac{1}{8} - \frac{1}{20} = \frac{3}{40}

, so

a = \frac{1}{20}

.

Answer: D) 20 days

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