Janet Swifteagle wants to purchase at least 30 minutes of
computer time from a time-sharing computer company. She
needs at least 7 minutes of time from Computer A. The time-
sharing company will rent Computer A only if the user
agrees also to rent Computer B for at least 15 minutes. The
rent for Computer A is $500 per minute and the rent for
Computer B is $200 per minute. In order to minimize the
total cost, how much computer time should Janet buy on
each machine?