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

Explanation

Let the total work is `72` unit
Work done by `A` and `B` is`72/18 = 4` unit per day.
work done by `B` and `C` is `72/24 = 3` unit per day.
work done by` C `and `A` is`72/36 = 2'`unit per day.
Now `A, B` and `C` can do the work in `9/2` unit per day
Now time taken by `A, B` and `C` all together is `72*2/9 = 16` Hours