A and B can do a piece of work in 18 days. B and C in 24 days. A and C in 36 days. In what time can they do it all working together 

One day work (A and B) $=\frac{1}{18}$

One day work ( B and C) $=\frac{1}{24}$

One day work of (C and A) $=\frac{1}{36}$

$2(A+B+C)\text{'}s$one day work

$=\frac{1}{18}+\frac{1}{24}+\frac{1}{36}=\frac{9}{72}=\frac{1}{8}$

So one day work of $(A+B+C)=\frac{1}{16}$

So total work can be done by $(A+B+C)$  $=16\text{\hspace{0.17em}days}$