Hervenson is working two summer jobs, making $12 per hour at Mcdonald's and making $8 per hour washing cars. In a given week, he can work at most 20 total hours and must earn a minimum of $200. If Hervenson works 15 hours at McDonalds, determine the minimum number of whole hours washing cars that he must work to meet his requirements. Write a system of inequalities to describe Hervenson's situation.