SOLUTION: How do you find out what number to average it? I always get confused and have to use guess and check! Help?! His average grade on tests are 68%,78%,90&, and 91%. What score should

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Question 792087: How do you find out what number to average it? I always get confused and have to use guess and check! Help?! His average grade on tests are 68%,78%,90&, and 91%. What score should he score to have an average of at least 85%?
Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
You can put this solution on YOUR website!
His average grade on tests are 68%,78%,90&, and 91%.
What score should he score to have an average of at least 85%?
:
Let x = grade on the 5th test to have at least 85
%2868%2B78%2B90%2B91%2Bx%29%2F5 => 85
:
%28327%2Bx%29%2F5 => 85
Multiply both sides by 5
327 + x => 425
subtract 327 from both sides
x => 425 - 327
x => 98
:
He needs to get a 98 or greater to get at least an 85 average