Excercise 21: When Sight-Rite's three polishing machines, A, B, 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 B and C are only working 4200 lenses can be polished in one week. How many lenses can be polished in one week by each machine alone.

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