A car is driven at 30 mph until it has completed one mile and then is driven at 60 mph for a further mile.
What is the car's average speed?
At first glance it seems obvious that the car's average speed is 45 mph, but that is not correct!
Speed equals distance divided by time.
The distance travelled is clearly two miles.
At 30 mph the car takes two minutes to cover the first mile, and then at 60 mph takes a further minute to go the second mile. A total of three minutes.
So the car has averaged two thirds of a mile per minute, or more conventionally it has averaged 40 mph.