A can paint a house in 18 days and B can do it in 12 days. Together with C, they completed the work in 4 days. Then, how long does C alone take to do the job?

9 days  

Let total work = 36,

Efficiency of A + B + C = \(\frac{Total\; work}{No.\; of \;days}\)

                                  =- \(\frac{36}{4}\) = 9

Efficiency of C = eff(A + B + C) - eff(A) - eff(B) = 9 - 2 - 3 = 4

Time taken by C alone = \(\frac{Total\; work}{Efficiency}\)

                               = \(\frac{36}{4}\) = 9 days