You can put this solution on YOUR website! If I average 20 miles per hour on the way to a show and 30 miles per hour on the way back, what was my average speed for the entire trip?
25 is not the average, because more time is spent going 20 mph than going 30.
Avg = 2*20*30/(20+30) = 24 mph
Suppose the distance from you to the show is 60 miles (we can choose such a number since the distances will all cancel out). Then, it takes 3 hours to get there and 2 hours to return back, so 5 hours total. You drove 120 miles total, so the average speed is 120 mi/5 hr = 24 mph.