#### Answer

$\displaystyle \frac{n!}{p!q!}$

#### Work Step by Step

See: Permutations of Duplicate Items, page 699.
The number of permutations of $n$ items, where $p$ items are identical, $q$ items are identical, $r$ items are identical, and so on, is
$\displaystyle \frac{n!}{p!q!r!\ldots}$
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Fill the blank with
$\displaystyle \frac{n!}{p!q!}$