An unknown radioactive substance has a half-life of 124 years. How long will it take a 67 grams sample to degrade to 18 grams?

Answer 1

67(1/2)^(t/124) = 18
or
.5^(t/124) = .2686..
take log of both sides

(t/124)log .5 = log .2686..
t/124 = 1.896164...

t = appr 235 years

check:
after 124 years, there would be 33.5 g left
after 248 years , there would be 16.75 g
My answer is reasonable