Question

A(t)=934(1/2)t/30 for 20 years and 100 years

Answer 1

$\bf A(t)=934\left( \frac{1}{2} \right)^{\frac{t}{30}}\qquad \qquad \stackrel{t=20}{A(t)=934\left( \frac{1}{2} \right)^{\frac{20}{30}}\implies A(t)=934\left( \frac{1}{2} \right)^{\frac{2}{3}}} \\\\\\ \stackrel{t=100}{A(t)=934\left( \frac{1}{2} \right)^{\frac{100}{30}}\implies A(t)=934\left( \frac{1}{2} \right)^{\frac{10}{3}}}$

nothing to it, t = year, and  you can just plug that in your calculator.