How to approach this combinatorics problem? You have $12$ different flavors of ice-cream. You want to buy $5$ balls of ice-cream, but you want at least one to be made of chocolate and also you don't want more than $2$ balls per flavor. In how many ways can you can choose the $5$ balls?
 A: Calculate the coefficient of $x^5$ in $$(x^1+x^2)(x^0+x^1+x^2)^{11}$$
A: This is the same as no of solutions of $\sum_{i=1}^{12} x_i=5$ where $1≤x_1≤2$ and for the other $x_i$'s $0≤x_i≤2$ which is the same as coefficient of $x^5$ in $(x^1+x^2)(x^0+x^1+x^2)^{11}$
A: Decide if chocolate will be a double ($c=1$) or a single ($c=0$).
If $c=0$, take a chocolate ball.  Decide how many "doubles" $d\in\{0,1,2\}$ you want and make one of $11 \choose d$ specific choices regarding which non-chocolate flavors you want.  These $d$ colors and chocolate are now off limits, so choose $5-1-2d=4-2d$ colors from each of the remaining $12-d-1=11-d$ legal  options.  
If $c=1$, take two chocolate balls.  Decide how many "non-chocolate doubles"  $d\in\{0,1\}$ you want and make one of $11 \choose d$ choices regarding the non-chocolate flavors.  These $d$ colors and chocolate are now off limits, so choose $5-2-2d=3-2d$ flavors from each of the remaining $12-d-1=11-d$ legal options.
$$\sum_{d=0,1,2} {11 \choose d}{{11-d} \choose {4-2d}} + \sum_{d=0,1} {11 \choose d}{{11-d} \choose {3-2d}}$$
$$= {11 \choose 2}  + \sum_{d=0,1} {11 \choose d} \left[{{11-d} \choose {4-2d}} + {{11-d} \choose {3-2d}}\right]$$
which is $55  + [1*495 + 11*55]=1155$. 
A: You can also answer this using careful counting
Case by case basis splitting into exactly two pairs of flavors, one pair
of flavors and no pairs.
$$C_1,P_1,P_1,P_2,P_2 = 1*12*11$$
$$C_1,P_1,P_1,P_2,P_3=1*12*11*10/2!$$
$$C_1,P_1,P_2,P_3,P_4=1*12*11*10*9/4!$$
How many of these ways lead to more than $2$ chocolates?
If
$$P_1=C$$
which for the first case can happen $11$ ways, the second case can happen
$11*10/2!$ ways and the last case can't happen if just $P_1=C$
What about if $P_2=C$ the first case can happen in $11$ ways and the second
case can happen in 11*10/2! ways.
If $P_3,P_4=C$ then we don't get $3$ or more chocolates, so we are done!
The final answer is 
$$12*11+12*11*10/2! +12*11*10*9/4!-11*10/2!-11-11-11*10/2!=1155$$
A: I think the most obvious/straightforward way to solve this is to use combination 'with repetition'/'stars and bars' and then subtract all inapplicable cases. (https://en.wikipedia.org/wiki/Combination#Number_of_combinations_with_repetition). $\space$The formula for this for selecting $r$ from $n$ is ${{n+r-1}\choose r}$. $\space$(I like to think of this as selecting $r$ 'items' from the $n+r-1$ 'walls' and 'items' and then  letting the $n-1$ 'walls' 'fall into place.')   
Dividing into cases where there is one chocolate ball and more than one.
One chocolate ball of ice cream:
$11$ flavors left to choose from.  Is combination 'with repetition' with $n=11$ and $r=4$ and then subtract all inapplicable cases. (All cases - cases where there are $3$ of $1$ flavor and $1$ of another - cases where there are $4$ of one flavor):
${{11+4-1=14}\choose 4}-{11\choose 1}{10\choose 1}-{11\choose 1}=1001-110-11=880$
Two chocolate balls of ice cream:
$11$ flavors left to choose from.  Is combination 'with repetition' with $n=11$ and $r=3$ and then subtract all inapplicable cases. (All cases - cases where there are $3$ of $1$ flavor):
${{11+3-1=13}\choose 3}-{11\choose 1}=286-11=275$
For a grand total of $1155$.  
