Description for Correct answer:
(A + B)'s 1 day's work = 1 / 72
(B + C)'s 1 day's work = 1/120
(A + C)'s 1 day's work = 1 /90

2(A + B + C)'s 1 day's work =\( \Large\frac{1}{72} + \frac{1}{120} + \frac{1}{90} \)

(A + B + C)'s 1 day's work = \( \Large\frac{5 + 3 + 4 }{360 X 2} = \frac{12}{360 X 2} = \frac{1}{60} \)

A's 1 day's work = (A + B + C)'s 1 day's work - (B + C)'s 1 day's work
=\( \Large\frac{1}{60} - \frac{1}{120} = \frac{2-1}{120} = \frac{1}{120} \)

A alone can finish the work in 120 days.