A purchaser of transistors buys them in lots of 20. He randomly inspects 3 components from a lot and accepts the lot only if all 3 are nondefective. Among 100 lots that he can choose for further inspection of components, 30 of them have 10 defective and 10 nondefective transistors, and 70 of them have 5 defective and 15 nondefective transistors. Write an expression for the probability that a chosen lot is accepted.