Math, science
posted by Sam .
assume the substance has a halflife of 12 years and the initial
amount is 133 grams. How long will it be until only 40 % remains?

Math, science 
Reiny
1(1/2)^(t/12) = .4
2^(t/12) = .4
take log of both sides
(t/12) log 2= log .4
t = 12 log .4/log2 = appr. 15.9 years.
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