Algebrahard · Past Paper
A and B can do a piece of work in 12 days, B and C in 15 days, and C and A in 20 days. How long would A take to do the work alone?
A30 days
B20 days
C24 days
D40 days
✓ Correct Answer: A — 30 days
1/A+1/B=1/12, 1/B+1/C=1/15, 1/C+1/A=1/20. Adding gives 2(1/A+1/B+1/C)=1/5, so 1/A+1/B+1/C=1/10. 1/A = 1/10 - 1/15 = 1/30. So A takes 30 days.
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