Ashipment of 12 microwave ovens contains 4 defective units. a restaurant buys three of these units. what is the probability of the restaurant buying at least two nondefective units?

The correct answer is:

20/27 = 0.7407.

Explanation:

To find the probability that the restaurant buys at least 2 non-defective units, we find the probability that they purchase either 2 or 3 non-defective units:



They purchase 3 units total, so this is n.
We want the probability of either 2 or 3 non-defective units; this means r is 2 and r is 3.
In this case we want the probability that the units are non-defective.  Since there are 4 defective units, this means there are 12-4=8 non-defective units; this makes the probability of a non-defective unit 8/12 = 2/3.  This is p.

Using these, we have: