(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.
