Answer
$$3.1 \times 10^{-4}$$
Work Step by Step
We assume that rate of mass loss still (fixed or constant).
Then, total mass loss will be:
$\begin{aligned} \Delta M &=(d M / d t) \Delta t=\left(4.33 \times 10^{9} \mathrm{kg} / \mathrm{s}\right)\left(4.5 \times 10^{9} \mathrm{y}\right)\left(3.156 \times 10^{7} \mathrm{s} / \mathrm{y}\right) \\ &=6.15 \times 10^{26} \mathrm{kg} \end{aligned}$
The fraction lost is
$$
\frac{\Delta M}{M+\Delta M}=\frac{6.15 \times 10^{26} \mathrm{kg}}{2.0 \times 10^{30} \mathrm{kg}+6.15 \times 10^{26} \mathrm{kg}}=3.1 \times 10^{-4}
$$
