An element has two isotopes. One isotope has a percent abundance of 75.0% and a mass number of 25 amu. The second isotope has a percent abundance of 25.0% and a mass number of 28 amu. What is the average atomic mass of the element?

Explanation:

this one's pretty easy,  25 x 75/100

28 x 25/100

then add the answers :)

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