How do I calculate percentage when the average is given?

Here is the question.



In an exam, the average score of the class was 70 marks. If the average score of the candidates who failed was 40 marks and that of the passed students was 80 marks.



Find the pass percentage of the class?How do I calculate percentage when the average is given?
average score of the class= 70

let number of failed student = N1

number pass student = N2

total score of failed student = 40 N1

total score of passed student = 80 N2

Total score of class = 70(N1+N2)

Therefore 40N1+ 80N2 = 70N1+ 70N2

80N2 -70N2 = 70N1-40N2

10 N2 =30 N1

N1/N2 =10/30 =1/3

N1:N2 = 1:3

Pass % of class = 3/4 *100 =75Ans.How do I calculate percentage when the average is given?
Add them together and divide by the number of entries.How do I calculate percentage when the average is given?
let x = percentage that passed

then 100 - x = percent that failed (total pass + fail = 100 percent)

total score of those that failed = 40(100-x)

total score of those that passed = 80(x)

total class score = 70(100)

so 40(100-x) + 80(x) = 70(100)

4000 - 40x + 80x = 7000

40x = 3000

x = 75 percent