Okay, after some thinking I realized that using math you can make .999(repeating) equal one exactly, try to follow me, this is how: x=.999(repeating) 10x=9.999(repeating) 10x-x=9 9x=9 x=1 This is a paradox that mathematically using this logic makes perfect sense but keep in mind that no matter what .999(repeating) is not equal to 1, it is infinitely close but they are still not the same number, or at least they shouldn't be...
He is using basic algebra to create an equation. X is equal to 0.999 he then goes on to say 10x= 9.999 because you just move one 9 forward so 10x-x= 9.999 - 0.999 this is just a form scientific notation you learn this in school.