$\"\"$

\

The combination is $\"\"$ .

\

The combinations formula is $\"\"$ .

\

$\"\"$ (Apply combinations formula)

\

$\"\"$ ( $\"\"$ )

\

$\"\"$ (Expand $\"\"$ )

\

$\"\"$ (Cancel common terms)

\

$\"\"$ . $\"\"$

\

The total number of combinations are 15.