In mathematics, there are some properties of numbers that are handy to know. For example, the identity property of multiplication says that any number times one is equal to the original number. So, according to that property 5 x 1 = 5.
Because of the default on the credit card, Mark's credit ratings will be reduced because the card is his and it will be treated as though it was Mark himself who defaulted.
When he goes to apply for a car loan, the lender may refuse to issue the loan due to Mark's poorer credit rating and credit card default. If he does find a place where he can loan from even with the new credit rating, they will most probably charge him a much higher interest rate to compensate for the risk of loaning him money.