What is the probability that at least 1 defective computer chip will be found

A piece of electronic equipment contains 6 computer chips, 2 of which are defective. 3 chips are selected at random and inspected.

P(at least one defective) = 1 - P(none defective)

P(none defective) = (4 choose 3)(2 choose 0)/(6 choose 3)
  = 4*1/20
  = 1/5

So P(at least one defective) = 1 - (1/5) = 4/5