Question 284162
An average of 88% means that the sum of the four tests divided by 4 = .88 = 88%.
S = sum of the 4 scores
S/4 = 88
S = 4*88
S = 352
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We need to find the value of the 'next' test that will bring the average to 90%.
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(352+x)/5 = 90
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Multiply both sides by 5 to eliminate the denominator
352 + x = 450
x = 450 - 352 = 98
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So, the student needs to score 98% on the fifth test to pull the average up to 90%.
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Check by computing the average...
352 + 98 = 450
450 / 5 = 90
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Answer:  The student needs to score 98% on the fifth test to bring the average up to 90%.
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Done.