Nine students took a test and had an average of 85. Then Orville took the test, and the ten students (including Orville), had an average of 80. What was Orville's score?
A useful re-write of the average formula:
sum of items = (average) x (number of items)
For the first nine students,
sum = 85 x 9
80 x 9 = 720, and 5 x 9 = 45, so sum = 85 x 9 = 720 + 45 = 765
sum = 80 x 10 = 800
The difference in those two sums is Orville's score:
800 – 765 = 35
Poor Orville!