Question 1082872
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 <font color=red>needs a 95 on the next test</font> to get an average of exactly 80.