Let's try this problem based on average from AMC 10B, 2019.
Mr. Patrick teaches math to 15 students. He was grading tests and found that when he graded everyone's test except Payton's, the average grade for the class was 80 . After he graded Payton's test, the test average became 81. What was Payton's score on the test?
But try the problem first...
AMC 10A - 2015 - Problem No - 5
Challenges and Thrills in Pre-College Mathematics
If you are not getting correct answer you can start from here :
The average of a set of numbers is the value we get if we evenly distribute the total across all entries. So assume that the first 14 students each scored 80.
If Payton also scored an 80 the average would still be 80. In order to increase the overall average to 81 we need to add one more point to all of the scores, including Payton's. This means we need to add a total of 15 more points, so Payton needs 80+15 = 95 .