PLEASE HELP: Suzy grabs four marbles from a bag containing four white marbles and five blue marbles. Find the probability that exactly three of the marbles are blue.

Question

kropot72 · Answer

$$P(4\ blue)=\frac{\left(\begin{matrix}5 \ 3\end{matrix}\right)\left(\begin{matrix}4 \ 1\end{matrix}\right)}{\left(\begin{matrix}9 \ 4\end{matrix}\right)}$$

anonymous · Answer

awesome! thank you, i have been struggling forever trying to figure out how to do this

anonymous · Answer

in situations how do you know when to use nCr or nPr

kropot72 · Answer

np. As you can see, the hypergeometric probability distribution applies.
Regarding you question, nCr deals with combinations where the order of selection does not matter. nPr deals with permutations where the order of selection is important.
When calculating the hypergeometric probability, combinations are used.

anonymous · Answer

oh okay now i remember that! could i have you help me with another one really quick too?

kropot72 · Answer

So the calculation for your question can be written as:
$$P(3\ blue)=\frac{5C3\times4C1}{9C4}$$

anonymous · Answer

okay yeah thats how i typed it in the calculator

kropot72 · Answer

Sorry, I have to log out now. Please post as a new Question :)

anonymous · Answer

okay thanks anyways! :)