Question 1162733
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<pre>

    {{{1000/2}}} = 500 g  after 5 days  (one half-life period);


    {{{500/2}}} = 250 g  after 10 days  (two half-life periods);


    {{{250/2}}} = 125 g after 15 days    (three half-life periods);


    {{{125/2}}} = 62.5 g after 20 days   (four half-life periods).


<U>ANSWER</U>.  20 days, or 4 (four) half-life periods.
</pre>

From my post, &nbsp;you see that the answer can be obtained &nbsp;MENTALLY.


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See the lesson

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/logarithm/Radioactive-decay-problems.lesson>Radioactive decay problems</A> 

in this site.