Thread: Help.. Probability

1. A shipment of 10 television sets includes 3 that are defective. In how many ways can a hotel purchase 4 of these and relieve at least 2 defective sets.

- I did 7C2 and 3C2 and multiplied them and got 63, but the book says the ans. is 70. Not sure how to get that.

Originally Posted by msfynestrini

1. A shipment of 10 television sets includes 3 that are defective. In how many ways can a hotel purchase 4 of these and relieve at least 2 defective sets.

- I did 7C2 and 3C2 and multiplied them and got 63, but the book says the ans. is 70. Not sure how to get that.

You have computed the number of ways to get exactly 2 defective sets. The problem asks for the number of ways to get at least 2, i.e. 2 or 3. So all you have to do is find the number of ways to get exactly 3 defective sets and add that to your previous answer.