Half-life is a first-order reaction which means the rate of decay is constant. Using the supplied value of the half-life of 14c (5715 years), and that the piece of ancient charcoal contains only 20% as much of the radioactive carbon as a piece of modern charcoal, the calculated "age" or timespan when the tree got burned is 13269.819062281 or simply, 13270 years ago.