SOLUTION: Charlie has scored 89, 78, and 63 on his previous three tests. What score does he need on his next test so that his average (mean) is 75?

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Question 1202053: Charlie has scored 89, 78, and 63 on his previous three tests. What score does he need on his next test so that his average (mean) is 75?
Found 2 solutions by ikleyn, greenestamps:
Answer by ikleyn(52803)   (Show Source): You can put this solution on YOUR website!
.
Charlie has scored 89, 78, and 63 on his previous three tests.
What score does he need on his next test so that his average (mean) is 75?
~~~~~~~~~~~~~~~~~~~~~~ 

Let x be the unknown score on his fourth exam.


Charlie wants  

     = 75,

or

    89 + 78 + 63 + x = 4*75

    230 + x = 300

          x = 300 - 230

          x = 70.


ANSWER.  Charlie needs to get 70 on the 4th exam.

Solved.

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Answer by greenestamps(13200)   (Show Source): You can put this solution on YOUR website!


Often in a problem involving the average of numbers that are close together, it is easier to solve the problem by comparing the individual scores to the desired average, knowing that the "overs" and "unders" must balance.

89: 14 over
78: 3 over
63: 12 under

For those first three scores, the total is 17 over and 12 under, for a balance of 5 over. That means the fourth score should be 5 under.

ANSWER: 75-5 = 70
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