A and B together can complete a piece of work in 12 days and B and C together in 15 days. If A is twice as good a workman as C, then in how many days will be alone complete the same work ?

• Given that, (1/A)+(1/B)=1/12 --> (1),
  (1/B)+(1/C)=1/15 => 1/B=(1/15)-1/C --> (2) and
  A is twice as good as C means if A take X days to complete the work then C take 2*X days to complete same work.
  put (2) in (1) => (1/A)+(1/15)-(1/C)=1/12 => (1/A)-(1/C)=1/60 => (1/X)-(1/(2*X))=1/60 => X=30 =>A=30, C=60 and put one the values of Aor C in above (1) or (2) equations to get B values, By substituting , you'll get B=20
  
  Hence A=30, B=20 and C=60 days will take for each person to complete same work individually.
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