[Hey I have also been asked this question, A mouse dropping was found in a house in Ireland and had a 14C:12C ratio that was 57% of a modern day standard. Use this value to estimate the aprox date when the mouse visited the house? The half life of 14C is 5715 year I used the following equation. t = half life / 0.693 * ln ( A / Ao) therefore = 5715 / 0.693 * ln ( 100 / 570 therefore t = 4626.42 years? What do you guys think ???? Am I doing it right??