The amount left over after T years where P is initial amount is 
it halves each time so after t times of the 5000 years have elapsed, the amount is now (1/2)^t of the original amount
find how many half lives
10000/5000=2
(1/2)^2=1/4
1/4 of original amount after 10000 years
after 50000 years
50000/5000=10
(1/2)^10=1/1024
1/1024 of original amount after 50000 years
it dissapears when it attains an amount you deem is nigligible, so if you had 1 mol of subsutance, you might say it dissapears when it gets to 10^-23 or about 6.02 molecules
it halves each time so after t times of the 5000 years have elapsed, the amount is now (1/2)^t of the original amount
find how many half lives
10000/5000=2
(1/2)^2=1/4
1/4 of original amount after 10000 years
after 50000 years
50000/5000=10
(1/2)^10=1/1024
1/1024 of original amount after 50000 years
it dissapears when it attains an amount you deem is nigligible, so if you had 1 mol of subsutance, you might say it dissapears when it gets to 10^-23 or about 6.02 molecules