The element gallium has two stable isotopes. About 60% of the gallium atoms found are gallium-69, while the remaining 40% are gallium-71. The average atomic mass of gallium is about?

70 amu

Explanation:

The average atomic mass of an element is calculated by first finding the products of the atomic masses of the isotopes of that element and their respective percent abundances. The second/last step is finding the sum of those products, applying significant figures rules throughout. In this case, you would do the following:

(69 x 60%) + (71 x 40%) =  70 amu (with significant figures rules applied)

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