Rather than use that formula, look at the attachment and use the 4th equation.
Ending Amount = Beginning Amount / 2^n
where "n" is the number of half-lives. "n" = elapsed time / half-life
(Is 934 the beginning amount? Let's say it is.)
for 40 years, n=40/30 = 1.3333333...
Ending Amount = 934 / 2^1.3333333...
Ending Amount = 934 /
<span>
<span>
<span>
2.5198420998
</span>
</span>
</span>
Ending Amount =
<span>
<span>
<span>
370.66 grams
</span></span></span>
for 100 years, n=100/30 =
<span>
<span>
<span>
3.333333
</span>
</span>
</span>
...
Ending Amount = 934 / 2^<span>
<span>
3.333333
</span>
</span>
...
Ending Amount = 934 /
<span>
<span>
<span>
10.0793683992
</span>
</span>
</span>
Ending Amount =
<span>
<span>
<span>
92.66 grams
Source:
http://www.1728.org/halflife.htm
</span></span></span>