If one starts with a sample of 1.0 in a trillion 14C atoms,

   after this many years       this many 14C remain
-----------------------------------------------------------------
         0                              1.00       in a trillion

     5,730                              0.50

    11,460                              0.25
   
    17,190                              0.125

    22,920                              0.0625

    28,650                              0.03125

    34,380                              0.015625

    40,110                              0.0078125

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  So, if the measured value is 0.013 in a trillion,
then the age should be roughly 35,000 to 40,000 years ago.