How the heck did he figure this out in 1 minute?

I wish I could write on the screen, that would be infinitely more helpful. I resort to underlines and highlights, which make it slightly more difficult.

A reasonable assumption is one that is less likely to be susceptible to doubt. You can't doubt that Tigers are mammals. You can doubt that aggressive animals are not suitable to keep as pets, because some people have guard dogs (as mentioned in the previous lesson). A reasonable assumption is one that is air tight.