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Now I hope you have been exposed to the concept of weighted average;
If not here is a video

Now lets get our data;

Lets call our element x; Let its existence and abundance be divided in 100 parts

so

$10.01 $amu is present in 19.8parts

Total in this isotope is;

$10.01 *19.8 = 198.198 $ mass units

$11.01 $amu is present in 80.2parts

Total atomic mass in this isotope is;

$11.01 *80.2= 883.002 $ mass units

Now total number of mass units $= 883.002 +198.198= 1081.2$

But now the total number of parts$ = 100$

So all that's left is the weighted average;

$= 1081.2/100= 10.812 u$

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