What is the average speed of a bus whose speed is 40 km/m then 60 km/m then 70 km/m?

Average speed is total distance travelled divided by the total time taken. The total distance is equal to the sum of the speed multiplied by time for each of the three segments of travel. So it's impossible to answer your question without knowing either the time spent at each of the these three speeds, or the distance traveled at each of the three speeds.