.
Notice that = .
It means that half-life of iodine-131 is two times 11.4 days, or 2*11.4 = 22.8 days. ANSWER
Solved.
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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.
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By the way, the Internet sources give ANOTHER value for the Iodine-131 half-life: 8 days.
So, your input data is INCORRECT.
See, for example, these sources
https://en.wikipedia.org/wiki/Iodine-131
https://www.radioactivity.eu.com/site/pages/Iodine_131.htm