Answer
$ 0.412$
Work Step by Step
We need to use an exponential decay model such as:
$y= q( \dfrac{1}{2})^{t/24100}$
or, $q=\dfrac{y}{( \dfrac{1}{2})^{t/24100}} $
or, $q= y \times ( \dfrac{1}{2})^{-t/24100}$
The initial quantity after $1000$ years is:
$q= 0.4 \times ( \dfrac{1}{2})^{-1000/24100} \approx 0.412$
