The exponential decay function is shown as:
At = Ao*2^(-t/h)
Where:
At = Final amount
Ao = original amount
t = time
h = years for half-life to occur
We are not given the original and final amounts, but we are told that after the certain amount of time had passed, only 5% had remained. So if we were to isolate At and Ao in one side of the equation, such that: At/Ao = 0.05, we can solve for the missing variable, which is t.
Substituting:
At/Ao = 2^(-t/h)
0.05 = 2^(-t/20)
t = 86.44 years
Therefore, the pottery was made approximately 86.44 years ago.