Question 1126133
.
<pre>
The ratio  of 5 grams to 40 grams is  {{{5/40}}} = {{{1/8}}}, which is  (fortunately !) exactly  {{{(1/2)^3}}}.


It means that 40 grams will decay to 5 grams in the time which is exactly thrice the half-period, or exactly in 3*28 = 84 years.
</pre> 

Solved.


----------------


The instruction placed at the end of your post is VERY BAD advise on how to solve the given problem.


The much simpler way is that I showed in my post.