How would one sovle this problem?

Question

If Peter recieved grades of 88, 83, 75, and 80 on four tests in English, what is the minimum grade he has to score on the fifth test to have an average of at least 85 for the course?

Answer 1

the score needed is 99

you need to make up 3.5 points through 5 grades.

4 grades + 99 / 5 = 85

99 final answer

Answer 2

99

Previous average was 81.5, so must make up 3.5 points across 5 grades: 3.5*5=17.5, required above current average, or 99. 88+83+75+80+99/5=85.