Question 1162581
<br>
The amount remaining, y, of an initial amount x, is
{{{y = x((1/2)^n)}}}<br>
where n is the number of half lives.<br>
The number of half lives is the number of years, divided by the number of years in a half life.<br>
We are given x=1g and we are to find y:<br>
{{{y = 1((1/2)^(20/34.3))}}}<br>
Use a calculator....<br>
Very roughly, 20 years is something less than a half life, so your calculation should show an answer of slightly more than 0.5g remaining.  If you get a much different answer, you aren't doing the calculation correctly.<br>