This really boggled my mind! (algebra)

So, I'm not a mathematician, didn't go past highschool. I did think I was ok with algebra at least. I paused the video when the question was shown. My first answer was 3, but I put it on paper to check myself. Yep. It's still 3. Since circumference is πD, circle B has 3 times the diameter of circle A, therefore circle A has to make 3 revolutions around circle B, Wrong! It never registered with me that I had to add the radius of circle A into the calculations, effectively increasing the circumference of circle B. Some of you will have your eyes glaze over, that's ok. I'm sure someone here will say "You bonehead, this is obvious." Most folks don't use algebra/trig/geometry, etc. after they leave school. I don't use math for many practical applications, but I do find it interesting.
I'm just enough of a nerd that I'll try to solve equations in my head when I'm getting sleepy while driving to keep me alert. 
I had to finish the video to grasp the concept.

Feel free to put me on ignore!