form 2 equations
distance = rate * time
for the bus:-
320 = rt where r is the rate of the bus
For the car
280 = (r - 8)t
from first equation:- t = 320/r
so substituting in secod equation:-
280 = 320(r - 8)/ r
280r = 320r - 2560
40r = 2560
r = 2560/40 = 64
rate of bus = 64 mph