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I have a bit of a twist on a classic problem here.

I have a bag of $R$ red marbles, $G$ green marbles and $P$ pain in the purple marbles. I will draw $n$ marbles from the bag and am looking for the probability that I will have chosen exactly $r$ red marbles. So if the purple marbles were not special, I would have:

$Prob(r) = \dfrac{\binom{R}{r}\binom{G+P}{n-r}}{\binom{R+G+P}{n}}$

The change I need to make is that if a purple marble is drawn (or removed) from $P$, one of two things happens.

  • If there are any red marbles left, I will remove one of $R$ (I still count it towards $r$, but the "removed" marble does not count towards $n$)

  • If there are no red marbles left, I will remove one randomly from $G+P$ (again, not counting this extra "removed" marble towards $n$ BUT if it is a purple marble I will continue this step until a non-purple marble is removed)

This means that the total number of marbles taken from the bag will be the number drawn $n$ plus the number removed after getting a purple marble $p$.

I think that I will need to split this up based on both $p_{early}$ and the number of $p$ that happened before the bag ran out of red marbles, but I have had no luck so far. Any ideas on how to tackle this type of problem?

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  • $\begingroup$ What happens if the $n^{th}$ marble is purple? Do you draw another? Would it count if the next one is red? If you draw purple on number $5$, does the red you pull count as draw $6$? $\endgroup$ – Ross Millikan Mar 6 at 15:48
  • $\begingroup$ If the $n^{th}$ draw is purple I will draw another marble. If that extra drawn marble happens to also be purple I will draw again until I get a non-purple. If this draws a red I will count it. If I draw a purple on the second to last draw, that will remove (maybe) a red marble and then, after that, I will take my $n^{th}$ draw. I will update the question to make this clearer. $\endgroup$ – Hoog Mar 6 at 15:56
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As long as the desired number of red marbles is less than the total number you can just draw $n$ marbles without worrying about the purples, then replace all the purples you get with reds. You therefore want the chance you get $r$ marbles that are red or purple out of $n$ draws. The chance is then $$\frac {{R+P \choose r}{G \choose n-r}}{R+P+G \choose n}$$
If $r=R$ you just need to get at least $R$ of the purple and red marbles in the original draw. You need to sum over the possible numbers from $R$ to $R+P$ or $n$, whichever is less.

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  • $\begingroup$ Thank you @RossMillikan this is exactly what I needed to continue! $\endgroup$ – Hoog Mar 6 at 17:59
  • $\begingroup$ I just realized it is not correct because in your process when you draw a purple ball there are two purple+red balls removed before the next draw, while in mine there is only one. I’ll think some more. $\endgroup$ – Ross Millikan Mar 6 at 18:09

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