chemistry
posted by brittany .
If you accidentally spill phosphorus32 onto your shoe, how long would it take before 99.9 of the radioactive material has decayed so that you can safely wear the shoes again?

Is that 99.9% or some other value? You have no units.
k = 0.693/t_{1/2}
Substitute and solve for k, then substitute k into the equation below.
ln(No/N) = kt
No = some convenient number like 100.
N will be 0.1 if 99.9% of the atoms have decayed.
k from above.
Solve for t (in the same units as the half life is given).