Originally Posted by

**Jhevon** no! where is the 1/2 coming from, and why is 4 there? note, the $\displaystyle \frac 12 A_0 = A_0a^t$ equation was ONLY for half-life, nothing else. we did it to solve for a, it is NOT the equation we are working with. our equation is:

$\displaystyle \boxed{A(t) = A_0a^t}$

now you found a in the first part, you are now given $\displaystyle A_0 \mbox{ and }t$ and you are asked for how much is left, namely $\displaystyle A(t)$. plug in what you know, and solve for what you don't