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%

The  grandmother cell  is a hypothetical  neuronthat represents a complex but specific concept or object.[1]  it activates when a person "sees, hears, or otherwise sensibly discriminates"[2]  a specific entity, such as his or her grandmother. the term was in use at least as early as 1966 amongst staff and students in the department of experimental psychology, university of cambridge, england. a similar concept, that of the  gnostic neuron, was proposed two years later