Refer to the diagram shown below.
Define
x = distance, miles, that the speeding car travels before the police catch up.
t = time, hrs, before the police catch up.
Then
(80 m/h)*(t h) = (x mi)
80t = x (1)
(100 mi/h)*(t h) = (x+6 mi)
100t = x + 6 (2)
Subtract (1) from (2).
100t - 80t = x+6 - x
20t = 6
t = 6/20 = 0.3 h
From (1), obtain
x = 80*0.3 = 24 mi.
Answer: 24 miles