A lorry travelled 84km between two towns, the first 60km of road was untarred and the average speed over this part was 30km/h. If the average speed for the whole journey was 36km/h. Calculate the average speed over the good part of the road
Let x be the distance covered over the good part of the road.
The total distance covered is 84km. Therefore, the distance covered over the good part of the road is 84km - 60km = 24km.
The time taken to cover the first 60km is 60km / 30km/h = 2 hours.
The total time taken to cover the 84km journey is 84km / 36km/h = 2.33 hours.
Therefore, the time taken to cover the remaining 24km on the good part of the road is 2.33 hours - 2 hours = 0.33 hours.
Therefore, the average speed over the good part of the road is 24km / 0.33 hours = 72km/h.
Therefore, the average speed over the good part of the road is 72km/h.