[mlckb1]

Here is what the question reads:

plutonium 239 has a half life of 24100 years. plutonium is a serious radiation hazard and is present in spent uranium fuel from nuclear power plants. how many years does it take for 99% of the plutonium 239 to decay

I know you have to use the formula:
lnA_t/A_o =-kt

and that 99% is the ratio of A_t/A_o time 100.  Also that you can get k from the formula:
T_1/2 = .693/k

I just don't know where to go from here.  how exactly are you suppossed to find t when A_t  is zero.  i think i have all the elements to solving the problem i just need someone to point me in the right direction.

[Schrödinger]

At is not zero when 99% decay has taken place. Suppose that you start with 100 molecules of Pu, 99 molecules will have decayed. Which means 1 molecule remains undecayed. So, A_o=100 and A_t=1