[tex]\bf A(t)=934\left( \frac{1}{2} \right)^{\frac{t}{30}}\qquad \qquad \stackrel{t=20}{A(t)=934\left( \frac{1}{2} \right)^{\frac{20}{30}}\implies A(t)=934\left( \frac{1}{2} \right)^{\frac{2}{3}}}
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\stackrel{t=100}{A(t)=934\left( \frac{1}{2} \right)^{\frac{100}{30}}\implies A(t)=934\left( \frac{1}{2} \right)^{\frac{10}{3}}}[/tex]
nothing to it, t = year, and you can just plug that in your calculator.
