A and B can do a piece of work in 36 days, B and C can do a it in 60 days, A and C can do it in 45 days. C alone can do it in :
[A]90 days
[B]120 days
[C]150 days
[D]180 days

180 days
(A + B)’s 1 day’s work $= \frac{1}{36}$
(B + C)’s 1 day’s work $= \frac{1}{60}$
(C + A)’s 1 day’s work $= \frac{1}{45}$
2(A + B + C)’s 1 day’s work $= \frac{1}{36}+\frac{1}{60}+\frac{1}{45} = \frac{5+3+4}{180} = \frac{1}{15}$
∴ (A + B + C)’s 1 day’s work $= \frac{1}{30}$
∴ c’s 1 day’s work $= \frac{1}{30}-\frac{1}{36} = \frac{6-5}{180} = \frac{1}{180}$