SOLUTION: (3.) The average of your first three test grades is 78. What grade must you get on your fourth and final test to make your average 80?

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Question 498700: (3.) The average of your first three test grades is 78. What grade must you get on your fourth and final test to make your average 80?

Answer by Earlsdon(6294) About Me  (Show Source):
You can put this solution on YOUR website!
First, let's find the sum of the first three test grades, which we'll call a, b, and c.
%28a%2Bb%2Bc%29%2F3+=+78 this is the average of the first three grades. Multiply both sides by 3.
a%2Bb%2Bc+=+234 Now let's call the fourth test grade x, so...
%28a%2Bb%2Bc%2Bx%29%2F4+=+80 This is the desired average of the four test grades. But a%2Bb%2Bc+=+234 so substitute:
%28234%2Bx%29%2F4+=+80 Multiply both sides by 4.
234%2Bx+=+320 Subtract 234 from both sides.
highlight%28x+=+86%29
You will need to make a grade of 86 on the fourth test to obtain a final average of 80.