If an element X has isotopes of and relative abundance respectively (note that the sum of relative abundances is always 1) then the relative atomic mass ( ) is given by:

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As a rough check you can check the location of 50, 51.9 and 53 on a number line from 50 to 53. Note that 51.9 lies about 2/3 of the way along it thus it would follow that it will be roughly 1/3 of 50 and 2/3 of 53.

In your case

Solve for . Express in whichever method your teacher wants.