Answer:
He has to pay at least $175.
Step-by-step explanation:
As he wants to charge $200 airline ticket cost to his credit card without exceeding the credit line of $500, his available limit at the time the airline ticket is charged should be $200 or above; or the outstanding charge should be $300 or below ( that is, $500 - $200).
As his outstanding charge is $475, he needs to pay at least $175 ( that is, $475 - $300) in order to bring his available limit from $25 ( that is $500 - $475) to at least $200 so that the airline ticket cost can be charge to his credit card without going beyond the given limit of $500.