An inspector samples 2 chips from a box containing 13 chips. If the box contains 4 defective chips, how many ways can he select a sample so that at least one of the sampled chips is defective?

Answer 1

1st chip probably of defective: 4/13

if defective chosen: 2nd chip probability of defective = 3/12

if good chosen: 2nd chip probability of defective = 4/12.

multiply proabilities for consecutive picks.