If a radioactive substance reduces to 1/16th of its original mass in 40 days, what is its half-life ?

If a radioactive substance reduces to 1/16th of its original mass in 40 days, what is its half-life ?

Explanation

The substance reduces to (1/16) = (1/2)^4 of its original mass.  


This means 4 half-lives = 40 days  


So, 1 half-life = 40 / 4 = 10 days