@quacker88 Hi! Thanks for the helpful explanation because I had the same question victorioussheep did. It's just when I tried to solve the problem you gave, I got the answer B/(B-A).
Simplifying (1/A-1/B), I get (B-A/AB)t=1 and so t= AB/B-A.
t is the time it takes for the first runner to catch up with the second runner. So the number of laps the first runner runs is AB/B-A divided by A which is B/B-A.
Also, I think how Prof. Loh got A/A-B was by solving for the number of laps the trainer or the second runner was running.
I don't know if this is correct, but I hope you can look over my work. Thanks!
Here's another way to write the solution.
The bird is traveling at 30 miles per hour no matter what. The bird never slows down or speeds up. Because of this, the bird must be traveling at a constant speed. Since it's traveling at a constant speed, we can just find the distance by multiplying 30 by the time, which is 72/18=4. The answer is 120.