This is an example from Carol Ash's, "Probability Tutoring Book" which I don't understand.
You have 40 white, 50 red and 60 black balls. Pick 20 without replacement. Find the probability of getting 10 white, 4 red and 6 black.
She gives the answer $\frac{\binom{40}{10}\binom{50}{4}\binom{60}{6}}{\binom{150}{20}}$
She makes the point that it doesn't matter if the balls are distinguishable or not.
Why? Could someone explain this question from a combinatorics point of view?
Bonus: Could generating functions be used to solve part of this?
EDIT
I did a quick review of my discrete math notes from last year and this is how I would think to approach the problem.
Total number of possibilities would be the solution to $b+r+w=20$, namely $D(3,20)=\binom{22}{2}$, the number of ways to put 20 balls into slots labeled white, red, black.
Then I would take the number of permutations of 20 distinct balls divided by the desired number of white, red and black balls.
So I would get $\frac{\frac{!20}{!10!4!6}}{\binom{22}{2}}$. Does this seem correct? It's very different on the face from the given answer.
EDIT 2
What you’re missing is that even if there is nothing to distinguish one white ball from another, there are still 99 different white balls in the bag.
That gets to the heart of what I'm missing here. If I were asked, without connection to probability, how many sets of 20 balls I can get from 99 indistinguishable white balls and 1 red ball I would answer 2, like you said. But that would be incorrect because, in reality, there are 99 different white balls in the bag, as you pointed out. I was implicitly thinking of making a list of 20 from the options white or red with at most one appearance of red in the list, but that's a fundamentally different problem.
If I restated the problem as pick a committee of 20 from 99 men and one woman I wouldn't have made that mistake, because it's obvious that even though they're all "men", each man is distinguishable from each other man.
So in summary, it's important to not get confused between
- How many unordered lists of length $k$ you can make from a set of $n$ traits / types ( such as white, red, black, or man, woman, child ) which is $D(n,k)$
- Picking sets / committees of size $k$ from a set of $n$ items where some items might have a trait that seems to make them indistinguishable, such as white, red and black balls.