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 working together?
Given,
A and B can do a piece of work in = 18 days
Work done by A and B in 1 day = 
B and C can do a piece of work in = 24 days
Work done by C and B in 1 day = 
A and c can do a piece of work in = 36 days
Work done by A and C in 1 day = 
Now adding, we get,
2(A+B+C)’s 1 day work = 
A+B+C one day work = 
∴ A, B and C can finish the work in = 
7