Well, I need the decay rate (half-life) of C-14, as well as how much C-14 is in a living person. Both are figures that I just plain do not know. Did they not give you those? I suppose you could look them up...
Asumming you had those figures, you could set up a proportion:
(53/100)=(X/(the amount of C-14 in a living person)). You would then cross multiply and solve accordingly (I'm assuming you can do that decently).
Once you have X, subtract that value from the amount of C-14 in a living person. After that, simply divide by the decay rate of C-14. You should get your value- which !at or !at not be a nice and clean number, but that doesn't mean it's wrong if it's not. You might need a Calculator, and I would also check afterward.
Hope this helps!!