If A and B together can complete a work in 12 days, A and C together in 15 days and B and C together in 18 days, then how long they can do the complete work

A & B one day work = $\frac{1}{12}$

A & C one day work = $\frac{1}{15}$

B & C one day work = $\frac{1}{18}$

A, B & C's one day work = $\frac{1}{2}(\frac{1}{12}+\frac{1}{15}+\frac{1}{18})$

$$\begin{gathered} =\frac{1}{2}\left(\frac{15+12+10}{180}\right) \\ =\frac{37}{360} \end{gathered}$$ 

A, B & C can complete the work in $=\frac{360}{37}=9.7 days$