SOLUTION: Lucy scored a 85,64, and 76 on her math exams. What score must Lucy obtain on the next math test to have an average of exactly 80?

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Question 1082872: Lucy scored a 85,64, and 76 on her math exams. What score must Lucy obtain on the next math test to have an average of exactly 80?
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
Let x be the score on that fourth test.

The four test scores are: {85, 64, 76, x}

They add up to: 85+64+76+x = 225+x

Divide this sum by 4 since there are four tests

(sum)/4 = (225+x)/4

This expression is the average. We want the average to be 80, so set that expression equal to 80 and solve for x

(225+x)/4 = 80

4*(225+x)/4 = 4*80

225+x = 320

x+225 = 320

x+225-225 = 320-225

x = 95

Lucy needs a 95 on the next test to get an average of exactly 80.