Question

one of six radios in a crate is known to be defective. the radios are tested one at a time. if the first two radios are not defective, what is the probability that the third radio tested is defective.

Answer 1

You do not need to memorize or look up formulas to answer this. Just use basic logic.

There is still one defective radio among the four remaining untested radios. What does that tell you?