If a radioactive substance reduces to 1/16th of its original mass in 40 days, what is its half-life ?
Answer: 10 days
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
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