# Math Help - Series Problem

1. ## Series Problem

"A Shipment of 10 ovens has 3 defective. How many ways can a company buy 4 of these and receive A) All good units B) two good units C) at least two good units"

"Class is given 8 study problems, from which 5 will be part of upcoming exam. A student knows how to solve 6 of the problems, find probability that student can answer A) all five questions B) exactly 4 questions"

I am not sure If i have the right answers to these problems at all, and not sure how to do the last two at all.

2. ## Re: Series Problem

Originally Posted by thomasbu
I am not sure If i have the right answers to these problems at all, and not sure how to do the last two at all.
Why don't you show your attempt and answers to the first problem?

Originally Posted by thomasbu
"Class is given 8 study problems, from which 5 will be part of upcoming exam. A student knows how to solve 6 of the problems, find probability that student can answer A) all five questions B) exactly 4 questions"
Suppose the set of all questions and the subset of five exam questions are fixed. The sample space consists of subsets with 6 questions that the student knows. Obviously, there are ${8\choose 6}$ such subsets.

In how many ways can we select 6 questions out of 8 so that all five exam questions are there? We have to select the 5 exam questions (only one option), and the remaining 6th question can be any of the 3 non-exam questions.

In how many ways can we select 6 questions out of 8 so that exactly 4 exam questions are there? We need to select 4 questions out of the 5 exam questions, and select the remaining 2 out of 3 non-exam questions.