Multiple choice

A can do a piece of work in 15 days. B is 25% more efficient than A and C is 40% more efficient than B. A and C work together for 3 days and then C leaves. A and B together will complete the remaining work in:

  1. $2.5$
  2. $3$
  3. $3.5$
  4. $4$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A's rate is 1/15 work/day. B is 25% more efficient, so B's rate is 1.25 × (1/15) = 1/12 work/day. C is 40% more efficient than B, so C's rate is 1.4 × (1/12) = 7/60 work/day. A and C together for 3 days complete 3 × (1/15 + 7/60) = 33/60 work. Remaining work is 27/60. A and B together have rate 1/15 + 1/12 = 9/60. Time needed is (27/60)/(9/60) = 3 days.

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