How to calculate the start concentration for a first order reaction?
For a first order reaction $\ce{A -&gt;[k]P}$, the rate constant is given by

$$k[A] = - \frac{d[A]}{dt} $$

So, at any time $t$, the concentation $[A]_t$ is 

$$[A]_t = [A]_0 \cdot e^{-kt}$$

At the half-life time $t_{1/2}$, $[A]_t = \frac{1}{2}[A]_0$, isn't it?

Would you agree that we can calculate $k$ using $k = \frac{ln 2}{t_{1/2}}\ $?

Knowing $k$ and $[A]_t$, calculation of $[A]_0$ doesn't seem impossible now ;).