Which fraction of a sr-20 sample remains unchanged after 87.3 years?

First off there are some issues in your questions but I can still help in answering this type of problem. First, there is no Sr-20. The 4 abundant form includes Sr-84, Sr-88, Sr-87, Sr-86, Sr-84. 
More importantly, the half life of strontium must be given. 

For demonstration sake, let us assume Sr-85 is the isotope. It has a half-life of 64.853 days and I'm assuming the units of 87.3 is in days. 

A/Ao = e^-(ln2/t)T

where A- final amount Ao -initial amount t- halflife T-total time elapsed

% = e^-(ln2/64.853)(87.3) = 0.3933 or 39.33%

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