Answer:
100
Explanation:
Let the unknown score be x.
To find the average, you add all the scores and divide by the number of tests. Let's add all the scores including x:
89 + 95 + 76 + x = 260 + x = x + 260
All 4 scores, including the unknown one, add to x + 260.
To find the average, you divide the sum of the scores by the number of grades, which is 4 in this case.
The average is
(x + 260)/4
You want the average to be 90, so set the average equal to 90 and solve for x.
(x + 260)/4 = 90
Multiply both sides by 4.
x + 260 = 360
Subtract 260 from both sides.
x = 100
You must get a 100 on the next test.
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