Answer
$C(18,9)=48,620$
Work Step by Step
Using the combination formula, we obtain:
$C(n,r)=\frac{n!}{(n-r)!r!}$
$C(18,9)=\frac{18!}{(18-9)!9!}=\frac{18!}{9!9!}=\frac{18\cdot17\cdot16\cdot15\cdot14\cdot13\cdot12\cdot11\cdot10\cdot9!}{9!9\cdot8\cdot7\cdot6\cdot5\cdot4\cdot3\cdot2\cdot1}=48,620$