Finding the time on the walkway
Andrew: 6 km/h + 4 km/h = 10 km/h
500 m at 10 km/h = $\frac12$ km at 10 km/h takes $\frac1{20}$ of an hour.
Bill: 4 km/h
in $\frac1{20}$ of an hour, Bill travels 4000 m $\div$ 20 = 200 m
500 m $-$ 200 m = 300 m
Using proportion
Andrew: 6 km/h + 4 km/h = 10 km/h
Bill: 4 km/h
Distance and speed are proportional. Therefore, since Bill travels at $\frac{4}{10} = \frac{2}{5}$ of the speed of Andrew, he must travel $\frac{2}{5}$ of the distance. $\frac{2}{5}$ of 500 m is 200 m, so Bill is 300 m behind.