Posted by brittany on Thursday, May 12, 2011 at 5:23pm.
Is that 99.9% or some other value? You have no units.
k = 0.693/t1/2
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).
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