An incoming lot of silicon wafers is to be inspected for defectives by an engineers in a microchip manufacturing plant. In a tray containing 20 wafers, assume 5 are defectives. Four defective wafers are to be randomly selected for the inspection. Find the probability that -
(a) All three are nondefective.
(b) At least 1 is nondefective.
(c) Given that at least 1 is defective, what is the probability that all three are defective.