A box contains nine bulbs out of which 4 are defective. If four bulbs are chosen at random, find the probability that exactly three bulbs are good. Choose the correct choice from the following choices:
A) 20/65
B) 20/63
C) 20/68
D) 20/66
E) 20/29
Required probability   = (5C3 . 4C1)/9C4   = (10 * 4)/126   = 20/63   Answer:20/63
<result>B</result>
A box contains nine bulbs out of which 4 are defective. If four bulbs are chosen at random, find the probability that exactly three bulbs are good.