a student has scores of 74%, 78% and 70% on three exams. What score is needed on a fourth exam for the student to earn an average of 80%?

(74 + 78 + 70 + x) / 4 = 80

Divide each number by 4.

18.5 + 19.5 + 17.5 + x = 80

Add the numbers you know so far.

18.5 + 19.5 + 17.5 = 55.5

You need 24.5.

Now, since we divided all the others by 4, we have to multiply 24.5 by 4 to see what the exam score would be.

24.5 * 4 = 98.

To get an average of 80%, the student would need to get a 98 on their fourth exam.