A, B and C together can do a piece of work in 6 days. A and B together can do the same work in 12 days. B alone can do the same work in 18 days. In how many days can A and C together do the same work?

9 days

A + B + C = 6 days,

A + B = 12 days,

B = 18 days,

⇒ A + B + C = 6  (Efficiency)

⇒ A + C + 2 = 6,

⇒ A + C = 4,

⇒ Time required by A + C to complete the work = \(\frac{36}{4}\) = 9 days,   ..(\(\frac{Work}{Efficiency}\) = Time)