Question

a car travels at a speed of 80 km/h for 2½ hours from town P to town Q. Then the car increases its speed by 25% and maintains that speed for a further 150 km from town Q to town R. Calculate the average speed for the whole journey.

Answer 1

So this is not too hard, as with any math problem we write what we know.

Starting speed: 80km/h
Time from P to Q: 2.5h
Increase in speed: 20km/h (25% is dividing by 4)
Distance from Q to R: 150 kilometers

So let's find the speed and distance for both legs of the trip.

Speed for P to Q: 80km/h
Distance from PtoQ: 200km (2.5h * 80km/h)
Speed from Q to R: 100km/h (80 +20)
Distance from QtoR: 150km.

Now we multiply the speeds by their respective distances add them and then divide by the total distance.

((80 *200) + (100 * 150))/350 = ~88.57143