Question 1101006
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Here is an alternative solution method that I like to use when the numbers being averaged are all close together.  Try it and see if it "works" for you.<br>
With this method, we compare each of the known values to the known average to find how far away they are; from that we can determine how far from the average the missing value has to be.<br>
In this example, the final average is 28.5.<br>
We have three values of 27; each of them is 1.5 below the average; so together those three values are 4.5 below the average.<br>
We also have three values of 29; each of the is 0.5 above the average; so together those three value are 1.5 above the average.<br>
Then the "balance" for the six known value is 3 below the average.<br>
That means the missing value must be 3 above the average: 28.5+3 = 31.5.