(a)In a certain examination you are required to answer six out of nine questions.Find the number of ways that you can choose the six questions.
We have nine questions to answer. But we need only six questions to answer. We can do this using the combinations.
Finally what we need is to find how many ways that 6 questions can be selected out of 9 questions.
The answer is `^9C_6 = (9!)/(6!xx3!) = 84`
So there are 84 ways to select six questions.