Question 1124495
d=k*s^2
200=k*60^2
200/3600=k
k=1/18 or 0.057

50=k*s^2
50/(1/18)=s^2
900=s^2
s=30 mph

This makes sense, because as the speed doubles (30 to 60) the stopping distance should increase by a factor of 4 (50 to 200).