AVG on tests = (earned points)/(number of points possible). For 4 tests, the best possible score is 400 points (100 each).
.83 = x/400 and x = 400*.83 = 332 points.
Call the points you'll receive on the 5th test y. The total possible points in 5 tests is 500. (332+y)/500 = .9
332+y = 450
y = 118
You'd need 118 points out of 100 to achieve a 90%. Unless there's extra credit, you cannot achieve 90%.
The best possible grade you can achieve is 332+100 = 432/500 = .864 - 86.4%
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