A shipment of 12 microwave ovens contains three defective units. A vending company has ordered four units, and because each has identical packaging, the selection will be random. What is the probability that (a) all four units are good, (b) exactly two units are good, and (c) at least two units are good?

Answer:a) 54/55

b) 100/110

c) 99/110

Step-by-step explanation:

a) Probability of ordering 4 = 9/12*3/11*2/10*1/9 = 54/11880

Probability of 4 good units = 4 * 54/11880

= 216/11880

1/55

1 - (1/55) = 54/55

b) Probability of 2 good units = 2 * 54 / (11880)

= 108/11880

= 1/110

1 - (1/110) = 100/110

c) Probability of exactly 2 units not good = 1 - (100/110) = 99/110