Question 1195486
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A typical algebraic solution....<br>
(12+18+21+c+13)/5 = 17
12+18+21+c+13 = 85
64+c = 85
c = 21<br>
That method for solving the problem is fine since the numbers are small, so adding the numbers is an easy task.<br>
If the numbers are much larger but still close together, an often useful method for solving a problem like this is to compare each number to the desired mean, knowing that the "overs" and "unders" have to balance.<br>
For this example, with the average being 17....
12: 5 under
18: 1 over
21: 4 over
13: 4 under<br>
Note the first three numbers balance; overall the four scores are 4 under the desired average.<br>
That means c must be 4 over the desired average.  So<br>
ANSWER: c = 17+4 = 21<br>