Picture below. I really don't understand any of this...

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anonymous · Answer

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aaronq · Answer

It's a weighted average.

It's identical as to how you would calculate your grades.

Say you for a class you have 2 assignments (each worth 30%) and an exam (worth 40%);  they add up to 100%.

You get 75 out of 100 on one assignment, 80 on the other and 70 out of 100 in the exam.

Your grade for that class is the weighted average, a sum of each mark times it's worth:

$Total~Grade=\sum individual~mark*Worth$

$Total~Grade=\dfrac{75}{100}*30\%*+\dfrac{80}{100}*30\%+\dfrac{70}{100}40\%=74.5\%$


For isotopes it's: 

$Averaged~Mass=\sum individual ~mass*\%~abundance$