Question 1130838
Description tells you that the half-life is 4 hours.


Without creating a function,
<pre>
TIME(hours)      miligrams
0                 200
4                 100
8                  50
12                 25
</pre>
40 milligrams would remain and new dose needed some time between 8 and 12 hours.  Probably near about 9 hours.



Starting through this would help get a more accurate result:
{{{y=200*(1/2)^(t/4)}}}

Let y=40, take logs of both sides to solve for t:


{{{t=4*log((5))/log((2))}}}


{{{highlight_green(t=9.29*hours)}}}