A contractor and a gravel truck driver arranged to meet at a sandpit 9 km away. In travelling to the oit, the gravel truck's average speed was 25 km/h slower than that of the contractor's car. The gravel truck took 4 minutes longer than the car to reach the gravel sandpit, as modelled by the equation 9/x-25 - 9/x = 4/60 to the nearest km/h , what is the average speed of the car