You have just graduated from college and purchased a car for $5000. your credit limit is $10,000. assume that you make no payments and purchase nothing more and that there are no other fees. the monthly interest rate is 8%. what is the growth rate of your credit card balance per month (as a decimal)?

Monthly interest rate of 8% (APR=96%) would be illegal in most states and actually exceed the "criminal rate" of 60% in Canada!

The monthly growth rate of the balance, assuming no fees, no payments, and no additional purchases is exactly 8% per MONTH.
For example, a 5000 loan grows by $400 the first month, $432 the next month, and so on.  He will be just a touch below his credit limit at the end of 9 months (5000*1.08^9)=9995!
Also, most card issuers will even increase the interest rate after a number of months (e.g. 6) of zero payment, which makes the situation worse!