The half-life of Pu-240 is 6,561 years.
Half life can be modeled by the function
f(x) = P(0.5)^x
P is our starting amount.
x is the number of half-lives required to equal f(x).
f(x) is the amount left after x half-lives.
9 = 27(0.5)^x
1/3 = 0.5^x
log(0.5)1/3 = x
x = 1.585
6.561 years * 1.585 half-lives = 10,399.185 years.
