Lily goes on a car journey. For the first 30 min. her average speed is 40 mph. She then stops for 15 min. She then completes the journey at an average speed of 60 mph. The total journey time is 1 h. What is her overall average speed?

average speed=total distance/time taken ; distance=speed x timeFor the first part of her motion the distance she travelled, d₁=40 x 1/2 (as 30 minutes represent half an hour), so d₁=20 milesFor the second part the distance she travells is 0 because she stops for 15 minutes.For the third part: d₃=60 x 1/4 (as 15 minutes represent a quarter of an hour), so d₃=15 miles.average speed=(20+0+15)/1=35 miles per hour