You have the next data:
[tex]s=\frac{d}{t}=\frac{400}{8}=50[/tex]
That is the constant s= 50 (the speed is the constant)
You can see that the equation that relates d and t could be find using the formula of the speed:
[tex]s=\frac{d}{t}[/tex]
you know that s=50:
[tex]50=\frac{d}{t}[/tex]
then:
[tex]50t=d[/tex]
and:
[tex]t=\frac{d}{50}[/tex]
