Explanation :
(A+B)'s 1 day's work = 1/8 ---------(1)
(B+C)'s 1 day's work = 1/12 ----------(2)
(A+B+C)'s 1 day's work = 1/6-----------(3)
∴ (A+C)'s 1 day's work =2(A+B+C)'s 1 day's work - [ (A+B)'s 1 day's work + (B+C)'s 1 day's work ]
$ = 2\times(1/6) -[(1/8) +(1/12)] = 1/8 $
So,A and C together will do the work in $ \cfrac{1}{(1/8)}\text{ days}$
= 8 days