A radioactive substance decays at a rate directly proportional to the amount present. If the substance is present in the amount of $\displaystyle Qo g$ initially $\displaystyle (t = 0)$, find the amount present at any time $\displaystyle t$.

I started off using the restricted growth model of

$\displaystyle dQ/dt = kQ(C-Q) $

That is about as far as I made it.