Answer: 34
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Work Shown:
S = sum of the first 8 numbers
S/8 = 40 is the mean of those first 8 numbers
S = 8*40
S = 320 so the first 8 numbers add to this value
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R = sum of the 12 remaining values
R/12 = 30
R = 12*30
R = 360 is what the other 12 values add to
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R+S = 360+320 = 680 is what all twenty values add to. Divide this by 20 to get the overall average
(R+S)/20 = 680/20 = 34