probability

[fredrick]

A bag contains 12 bread rolls, of which 8 are white and the remainder multigrain. Tony takes 2 bread rolls at random form the bad to eat.
The probability that at least 1 is a multigrain is:
[IMG]http://img137.imageshack.us/img137/6637/50978254iv2.png[/img]

how to get that answer?

[Mao]

however, you should keep in mind that this is hypergeometric probability distribution and is not on the course anymore.

[fredrick]

oh good, how do i identify if this is hypergeometric? all probability looks the same to me

[ed_saifa]

hypergeometric is without replacement

[Collin Li]

You would be fine to get this question and complete it. Questions that can be solved with hypergeometric are not automatically excluded. They are only excluded if hypergeometric is the only practical way of doing it (i.e.: a seven step without replacement problem - this is two steps).

Typically, you'd use a tree diagram, where the probability of picking multigrain is , and it changes in the next step.

The hard part is if it was multichoice, and the answer was as displayed (with combinatorics). I'm not sure whether you need to understand how to use combinatorics to represent these probabilities, but you shouldn't be too fazed by it. Basically it's saying: "this is the number of ways I can choose this from that, divided by the total number of combinations."

[fredrick]

thanks! i was gona draw a big tree but when i saw those combinatorics i stopped(it was a multichoice question). I guess i could of got the answer with the tree and compared it with the numerical value of the choices.

[Collin Li]

Certainly The only problem with that is if they made a question with pronumerals instead of numbers.