Posted by Josh on .
Exponential growth and decay
find the half life of a radioactive substance if 220 grams of the substance decays to 200 grams in 4 years?

Pre Calculus 
drwls,
Let the half life be T years
2^(4/T) = 200/220 = 0.9091
(4/T) log 2 = log 0.9091
4/T = 0.1375
T = 29.1 years