assume the substance has a half-life of 12 years and the initial

Question

amount is 133 grams. How long will it be until only 40 % remains?

Answer 1

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.