A, B and C can complete a piece of work in 24, 6 and 12 days respectively. Working together, they will complete the same work in

\par (A + B + C)'s one day's work =  \frac{1}{24}+\frac{1}{6}+\frac{1}{12} = \frac{7}{24}

\par So, A, B and C together will complete the job in \frac{24}{7} = 3\frac{3}{7} \par days