$$
\begin{array}{l}{\text { Selecting a Bill A box contains a } \$ 1 \text { bill, a } \$ 5 \text { bill, a }} \\ {\$ 10 \text { bill, and a } \$ 20 \text { bill. A bill is selected at random, and }} \\ {\text { it is not replaced; then a second bill is selected at random. }} \\ {\text { Draw a tree diagram and determine the sample space. }}\end{array}
$$