Could someone explain why ‘Average Speed’ is not equal to the Arithmetic Mean of Speeds?

Consider this problem from 5lb chapter 18 for example

Could someone explain why ‘Average Speed’ is not equal to the Arithmetic Mean of Speeds?

Consider this problem from 5lb chapter 18 for example

Average speed is the total distance divided by the total time taken to cover that distance. In this case, you can assume the distance to be 100 miles between the two towns. Time taken to travel at 50 mph would be 2 hours while that for 60 mph would be 1.67 hours. Now the total distance becomes 200 miles and the total time becomes 3.67. Dividing them will give the average speed, which is lesser than 55 mph.