Answer
$$
3.9 \times 10^{8} \mathrm{\ ly} \text { . }
$$
Work Step by Step
At current time, a separation between two galaxies $A$ and $B$ is given by
$$r_{\text {now }}=c \Delta t_{B}-c \Delta t_{A} \text { . Since } r_{\text {now }}=r_{\text {then }}+r_{\text {then }} \alpha \Delta t,$$ so we get
$$
r_{\text {then }}=\frac{r_{\text {now }}}{1+\alpha \Delta t}=3.9 \times 10^{8} \mathrm{\ ly} \text { . }
$$
