A can do a piece of work in = 10 days
B can do a piece of work in = 18 days
(A + B + C) can do a piece of work in = 4 days
∴ A’s 1 day work = 1/10
B’s 1 day work = 1/18
(A + B)’s 1 day work = 1/10 + 1/18 = (9 +5)/90
= 14/90 = 7/45
(A + B + C)’s 1 day work = ¼
∴ C’s 1 day work =1/4 – 7/45
= (45 – 28)/180 = 17/180
∴ C can do the piece of work in = 180/17 days
= 10. 10/17 days