Question 1102943
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The solution by the other tutor is valid.  However, with rounding error, the answer you will get with her solution (37.29mg) will NOT be correct to 2 decimal places.  If you use her method, you need to keep more decimal places in your intermediate calculations in order to get an answer that is correct to 2 decimal places.<br>
But her method is also a LOT more work than is necessary to get the answer; there is a MUCH easier and more straightforward way.<br>
The given information tells you immediately that the half life of the substance is 38 hours.
The given time of 54 hours is 54/38 half lives.
So the amount remaining after 54 hours, rounded to 2 decimal places, is
{{{100*(1/2)^(54/38) = 37.34}}}