Let’s say we travel 5 miles at 5 mph. That would take us an hour, right? Now we increase our speed to 10 mph. We will cover that 5 miles in half an hour, right? So is our average rate halfway between 5 and 10? No. For our average rate to be halfway between 5 and 10 we would need to travel at the two rates for the SAME time period. To cover the same distance, though, we will always spend less time at the faster rate. Thus the average is always skewed towards the slower rate. (Because we spend more time going slow). We will have traveled 10 miles in 1.5 hours for an average rate of 6.66mph. I hope this helps.