Problem of the week

Problem of the week: In a geometry course the grade is based on the average score on 6 tests, each worth 100 points. W. Orrier has an average of 88.5 on his first 4 tests. What is the lowest average he could obtain on his next 2 tests and still receive an "A" (an average of 90 or better)?

Show your work, or write an explanation.