A difficult sum for me

[Guest]

A and B can do a piece of work in 12 hours B and C in 12 hours and C and A in 15 hours. How long will they take if they work together? How long will each take to complete it independently??

[Guest]

Think of the rate per hour they work at when doing the Job.
If they work independantly.......
A takes 30 hours
B takes 20 hours
C takes 30 hours

If they all work together.......
Job done in 8.57 hours

[Alicelewis11]

Think of the rate per hour they work at when doing the Job.
If they work independantly.......
A takes 30 hours
B takes 20 hours
C takes 30 hours

If they all work together.......
Job done in 8.57 hours

How you can calculate this?????????????