Problem - 2535
There are $2$ white balls, $3$ red balls, and $1$ yellow balls in a jar. How many different ways are there to retrieve $3$ balls to form a line?
Given the involved numbers are relatively small, this problem can be solved using casework:
$\underline{no\ yellow\ ball}$
- If no white ball chosen, then all the three balls are red. There is only $1$ way.
- If $1$ white ball chosen, then we have $1$ white and $2$ red. There are $3$ ways.
- If $2$ white balls chosen, then we have $2$ white and $1$ red, There are $3$ ways.
$\underline{1\ yellow\ ball}$
- If no white ball chosen, then we have $1$ yellow and $2$ red. There are $3$ ways.
- If $1$ white ball chosen, the we have one color each. There are $3!=6$ ways.
- If $2$ white balls chosen, then we have $1$ yellow and $2$ white. There are $3$ ways.
Hence, the final answer is $1+3+3+3+6+3=\boxed{19}$ ways.