Math 55

Hello 

I need someone to solve this in discussion form-

  "Exercise 21 in Exercise Set 3.6 can be solved mentally after careful reading of the problem.  Explain how this can be done." 

"When Sight- Rite's three polishing machines, A, B, and C, are all working, 5700 lenses can be polished in one week. When only A and B are working, 3400 lenses can be polished in one week. When only B and C are working, 4200 lenses can be polished in one week. How many lenses can be polished in a week by each machine alone?