Given the formula A=A0e^(kt)
Where A0 is the original mass, and A is material present at t
The half life of Pu239 is 24100 years, calculate how long it takes for Pu239 to decay to 70% of its original mass.
k = 1/24100, so you want
e^(t/24100) = 0.70
you should expect something around 1/3 of a half-life, since only 30% of the material has decayed...