[CK]

all three pays 300..
but the boss only takes $250

so three of them paid $250
$250/3 = $ 83.3 repeating (what each of them paid)

waiter took $20
and the three ppl got $30 back (so $10 each)

so
each paid $83.3 repeating + $10 = $93.3 repeating

$93.3 repeating x 3 people + $20 that the waiter took = $300


the logic in the ques was just misleading..

well.....
you are kind of changing the question. each of the customers still paid $90, not $83.3.

a simple way to view this is all three customers paid $270 (90 * 3), and the waiter stole $20 from the manager. so the manager received $250 in total.