.
= .
Hence, 5 (five) half-lives are just over.
Thus, 20 minutes are 5 half-time periods.
It means that one single half-time period is 20/5 = 4 minutes.
Solved, answered and explained.
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To learn more on the subject, look into the lesson
- Radioactive decay problems
in this site.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
- ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lesson is the part of this online textbook under the topic "Logarithms".
Save the link to this online textbook together with its description
Free of charge online textbook in ALGEBRA-I
https://www.algebra.com/algebra/homework/quadratic/lessons/ALGEBRA-I-YOUR-ONLINE-TEXTBOOK.lesson
to your archive and use it when it is needed.