Let P be the speed of police car (in km/h), and let B be the speed of the bus. First condition says P - B = 40. (1) The second condition says= . (2) From (2), express P = and substitute it into equation (1). You will get = 40. Multiply both sides by 3 to rid of denominator. You will get 5B - 3B = 120, or 2B = 120. Then B = = 60. Thus we found that the bus' speed is 60 km/h. Then the police car speed is 60 + 40 = 100 km/h.