Math Credit Challenge

[MrRubix]

By the time you are at the halfway point having gone 20 mph, there is no speed you could go that would make the average speed 40 mph -- you'd have to somehow be at the destination the instant you hit the halfway mark given those assumptions.

You aren't given a distance, but that means we can assume one. Say it's 80 miles to the house. At the halfway mark you are said to be going 20 mph, which means it took you 2 hours to make it halfway (40 miles). For it to be an average speed of 40 mph at this point you'd have to cover the rest of the distance instantaneously, which would be the same as covering 80 miles in 2 hours, or 40 miles per hour in terms of average speed. As you can see, anything slower than instant travel will make the denominator larger, meaning a lesser average speed if you can't make the trip instantly.