Let the total score for first nine innings be 9x.
Score after the tenth inning will be 9x + 100
Now, the equation would be of the form:

9x/9 +8 = (9x + 100)/10
(9x + 72)/9 = (9x + 100)/10
90x + 720 = 81x + 900
90x minus 81x = 900 minus 720
9x = 180
x = 180/9
x=20

And the new average would be 20 + 8 = 28

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