A class of 30 students recently took a test.  If 20 students scored 80, 8 students scored 90, and 2 students scored 100, then what was the class average (the mean) on this test?
To find the class average, I need to add up all the test scores and divide by the number of students. I can use the fact that 20 students scored 80 to simplify the calculation. Instead of adding 80 twenty times, I can multiply 80 by 20 and get 1600. Similarly, I can multiply 90 by 8 and get 720, and multiply 100 by 2 and get 200. Now I have the total of all the test scores: 1600 + 720 + 200 = 2520. To find the average, I divide this by the number of students, which is 30. So the class average is 2520 / 30 = 84. 