math
posted by Jackie .
assume the substance has a halflife of 11 years and the initial amount is 126 grams.How long will it be until only 15 % remains?

1/2 = e^k(11)
ln .5 = 11 k
11 k = .5978
k = .05435
so
.15 = e^.05435 t
1.897 =  .05435 t
t = 34.9 years