I really need some help please

Question

A student received scores of 85 and 78 on the first two exams. All exams have a maximum score of 100. What are the possible scores the student could receive on the third exam so that the average of the three exams is at least 80Write your answer in interval notation

Answer 1

(85 + 78 + x)/3 = 80

85 + 78 + x = 3(80) = 240

x = 240 - 85 - 78 = 240 - 163

x = 77

You need at least a 77 to get an 80 average

85 is 80 + 5

78 = 80- 2

77 = 80 - 3

5 - 2 - 3 = 0