What a coincidence! i found the Grail in my backyard just last week. mine is the real one of course.

Let be the amount of carbon-14 remaining at time t, let be the original amount of carbon-14. since we lose carbon-14 in an exponential decay, we have that:

where is the rate of decay and is the time elapsed.

now, we know that where is the half-life

thus we have:

now assume we have 1 unit to begin with. then the A remaining after 2000 years (in the decimal form of a percentage) is given by:

now calculate that