To calculate percent abundance from relative abundance, multiply the relative abundance by 100%. For example, if the relative abundance is 0.34, the percent abundance is 34%.
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To calculate percent abundance from atomic mass and relative atomic mass, set one unknown to x and the other to 1-x. The first unknown can replace the x in the second unknown.
Therefore if [unknown 1 = a] and [unknown 2 = b] and a(x) + b(1-x) = the relative atomic mass, then
a(x) + b - bx = relative atomic mass.
Knowing the relative atomic mass and the values of a and b, you can insert them into the equation to solve for x.
Then from there, you can solve for the unknowns.
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Here is a link for a helpful video: https://youtu.be/6CfSyGd5Ry4