The important thing to note is that this is a ratio problem. So what I do is call up a random easy-to-use distance and calculate (total miles/total hours). For this, I'd use 120 miles as the distance. At 40 & 60mph, it takes 3+2 hours to travel the round trip 240 miles. 240/5=48
It doesnt matter what distance you use. He could drive 60 miles, 120 miles, 10 miles, 5 feet... the average speed round-trip will always be the same. You can try out different distances if you dont believe it.
Another thing I do is look at the answers first before starting the math. Sometimes the answer is blatantly obvious. if the guy travels at 50mph both ways, that's the average speed. however he travels at 40 mph, which he spends longer traveling at than the 60mph back, so the answer will be skewed a little under 50mph, toward the 40mph. if your possible choices are 40, 48, 50, 52, or 60, it has to be the 48. This obviously doesn't work too well if your options are 46,47,48,49 and 50
