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Question
A and B working together can mow a field in 56 days and with the help of C, they could have mowed it in 42 days. How long would C take by himself?
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Solution
(A+B) can mow a field in = 56 days
(A+B+C) can mow a field in = 42 days
∴ (A+B)'s 1 day work =`1/56`
(A+B+C)'s 1 day work =`1/42`
C's 1 day work =`1/42-1/56`
`=(4-3)/168`
`=1/168` 
L.C.M. = 2 × 7 × 3 × 4 = 168
∴ C can do the work in = 168 days
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