To pass a course with a B grade, a student must have an average of 80 points on 5 tests.

Question

On the first 4 tests, the student's scores were 75,83,86, and 78. What scores can the student receive on the fifth test to earn a B grade?

Answer 1

Getting an average of 80 on 5 tests is the same as getting 5 80's which is a total of 400 points.

Adding the test scores you already have gives you 322 points. 400-322=78 as shown.

Here's another way to look at the problem:

75 is -5 from 80

83 is +3 from 80

86 is +6 from 80

78 is -2 from 80

(-5)+(+3)+(+6)+(-2)=+2, which means you have 2 points extra in getting all 5 tests to be 80, therefore you can get a 78 on the last test and have these 2 extra points to add to 78 to make the 80 needed

Answer 2

(75+83+86+78+the fifth score)/5 should greater than or equals to 80, then the fifth score = 78, which means you at least should get 78 in order to receive a B grade.