(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