For this problem, we use the formula for radioactive decay which is expressed as follows:
An = Aoe^-kt
where An is the amount left after time t, Ao is the initial amount and k is a constant.
We can use the equation above to find the percent of the original substance at a given time. However, the time given here is equal to the half life of C-14. So the percent left would be 50% or half the original.