Mr. thomas has an outstanding balance of $128.75 and owes a copay of $30.00 for today’s visit. how much money should be requested from mr. thomas at check out?

$158.75, if the total balance is due at check out.

Explanation:

Mr. Thomas owes $128.75 and has just increased this by $30 for today's visit.  $128.75 (owed) plus $30 (today's visit) equals $158.75.  

It may be that the company will allow Mr. Thomas to pay at least 50% ($79.38) or 30% ($52.92), and then pay the rest later.