Drawing balls with a finite number of replacement I have to solve this problem:
"Suppose a box contains $18$ balls numbered $1–6$, three balls with each number. When $4$ balls are drawn without replacement, how many outcomes are possible?". (The order does not matter).
I can't find a simple formula for it.
I've tried in this way and I don't know if it is right way:

A random outcome could or could not have the number $1$.
If it has it, the outcome could be $111$ plus a number $2\le n \le 6$, or $11$ plus two numbers or $1$ plus three numbers.


*

*In the first case we have a total of ${{5}\choose{1}} = 5 $ outcomes.

*In the second case we have a total of ${{5}\choose{2}} + 5 = 15$ outcomes.

*In the last case we have a total of ${{5}\choose{3}} + 5 +5\times 4 = 35 $ outcomes.


Finally, if the outcomes does not have the number 1 we have a total of $ {{5}\choose{4}} + 5\times(4\times 3 + 4) + 5\times 4 + 5 = 110$.
So there are 165 possible outcomes. 

Is it right? If yes, there is a simpler and much more elegant way to prove it?
Thanks

One more way is to use a generating function. Consider
$$F(x) = (1+x+x^2+x^3)(1+x+x^2+x^3)(1+x+x^2+x^3)\cdots(1+x+x^2+x^3) = (1+x+x^2+x^3)^6$$
Looking at the first $(1 + x+ x^2 + x^3)$ term, we can think of the exponent of $x$ as representing the number of "1" balls we choose (i.e. $1=x^0 \rightarrow 0,\ x = x^1\rightarrow 1,\ x^2\rightarrow 2,\ x^3\rightarrow 3$). Correspondingly, the second term represents the number of "2" balls we choose, etc. Since the exponents in each term range from 0 to 3, we are restricted to choosing at most 3 of any type of ball. The answer to your question is then given by the coefficient of $x^4$ (since we are choosing a total of 4 balls) in the expansion of $F(x)$. A computer can easily confirm that the coefficient is 120, as given by the other answers.
This method is nice because it can be generalized to more complicated conditions fairly easily. For example, if you are only interested in the number of drawings where an even number of "1" balls are present, you can simply change the first term to be $(1 + x^2)$ (note the even exponents) and find the coefficient again.
