i think
let's say the 2 buses a and b. since b is slower than a in 11mil/hr we can set an equation of a=b+11km/hr
a is 636miles away from b in four hours. so we can calculate the speed.
speed=distance/time
so V=636mil/4hr=159mil/hr
so the speed of a is 159mile/hr and b's is 148mile/hr
