Question 1141321
P=Poe^-kt
when P/Po=0.5,
then 0.5=e^-kt
ln(0.5)=-k(5730)
-0.693=-5730k
k=+0.0001209

Let P=0.20Po
so P/Po=0.20
ln(0.20)=-0.0001209t=-1.609
so t=-1.609/-0.0001209
=13,308.52 years or 13309 years 
If rounding only at end, it will be 13308 years.

This is between 2 and 3 half-lives, which is where 20% is.