Multiple choice

A and B can do a piece of work in 24 days. B and C can do a piece of work in 30 days. C and A can do a piece of work in 40 days. How long would they all take to do the same work?

  1. 28 days

  2. 24 days

  3. 20 days

  4. 18 days
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

1/(A+B) = 1/24, 1/(B+C) = 1/30, 1/(C+A) = 1/40. Summing these: 2(1/A+1/B+1/C) = 1/24 + 1/30 + 1/40 = (5+4+3)/120 = 12/120 = 1/10. So 1/A+1/B+1/C = 1/20. They take 20 days.

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