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Exponential rate of decay per year "r" remains THE SAME during the entire decay process
(i.e. FOREVER, or, more precisely, until the last radioactive atom decayed :-).
It is equal to r = = = = = 0.666666... or 66.6666% of the mass per year.
By the way, when you formulate a problem like this one, you should tell in your question, which rate of decay do you mean:
exponential coefficient of decay or linear coefficient at the given time.
These are two different values and two different conceptions.
Without clarification, the meaning of the question is dark / unclear.
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What the tutor ankor@dixie-net.com wrote in his response, was not an answer to your question.
It was something V E R Y different. It was about the remained mass amount after 2 years.
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On radioactive decay problems, see the lesson
- Radioactive decay problems
in this site.