A can plough a field in 12 hrs  and B can do it in 4 hrs. Together with C, they completed the work in 2 hrs. Then, how long does C alone take to do the job?

6 hrs     

Let total work = 12,

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

                                  =- \(\frac{12}{2}\) = 6

Efficiency of C = eff(A + B + C) - eff(A) - eff(B) = 6 - 3 - 1 = 2

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

                               = \(\frac{12}{2}\) =6 hrs