# Number of arrangements of red, blue, and green balls in which a maximum of three balls of the same color are placed in the five slots

From the picture above I have five slots and a bag of balls (three) of color -red, blue and green...

Question

Now whenever I choose a ball, I note the colour and replace it in the bag, then I randomly keep choosing balls (independent event) until all five slots are filled and put them back in the bag..

How can I derive a formula to find all possible repetitive arrangements (exhaustive approach) with the condition that a MAXIMUM of THREE balls of the same color are in the five slots.

Note:

I have ask a similar question at Finding the Total number of permutation using a selective formula

In the accepted answer, there were five slots and five letters and all arrangements with a maximum of three letters were found using a pattern

$3-1-1: \binom5{1,2,2}\binom5{3,1,1}$
$2-2-1: \binom5{2,1,2}\binom5{2,2,1}$
$2-1-1-1: \binom5{1,3,1}\binom5{2,1,1,1}$
$1-1-1-1-1:\binom55\binom5{1,1,1,1,1}$

Now in this question the number of balls and slots varies

I would like to build from the pattern above answer using multinomials (since I don't want to write a new algorithm again)

• I don't think this is clear. What do the slots have to do with anything? Are you saying that when you note a color you write it in one of the slots, UNLESS doing so would violate the cap rule? But then...why not just ask "how many words on $\{R,B,G\}$ are there of length $5$ with no letter appearing more than $3$ times?" – lulu Aug 21 '16 at 19:18
• yes I would like "how many words on {R,B,G} are there of length 5 with no letter appearing more than 3 times?" – repzero Aug 21 '16 at 19:28
• @lulu note too order/arrangements counts – repzero Aug 21 '16 at 19:29

We will solve the problem: "how many words of length $5$ on $\{R,B,G\}$ are there in which no letter appears more than $3$ times".
Easier to work backwards. Without the cap rule there are $3^5$ possible words.
How many of these have exactly $4\;R's$? Well there are $5$ place to put the non-$R$, and $2$ options for what letter it is. Hence $10$. As there are three letter, there are exactly $30$ words of length $5$ in which some letter appears exactly $4$ times.
Now, of course there are eactly $3$ words in which a letter appears $5$ times.
Hence there are $30+3=33$ words of length $5$ in which some letter appears more than $3$ times.
It follows that there are $3^5-33=\fbox {210}$ words of length $5$ in which no letter appears more than $3$ times.