There is *NO* missing dollar. In the left-hand-side of the
equation, we have the manager keeping ($30 - $5) or $25. On the
right-hand-side of the equation, we have the men paying ($9 x 3)
minus $2 pocketed by the waiter, which is $25.
($30 - $5) = ($9 x 3) - $2
$25 = $25