Well, first of all I have to mention that the guy is an idiot and there is no mathematical explanation for being an idiot! A mathematical response as to what they did wrong is on the adding they added up the threes and then the ones WITH the ones… they didn’t count them separately like they are supposed to do. With the division they save the two for later and they’re not supposed to do that and they ended up getting the answer the fat guy wanted to get. Finally, the multiplication when wrong when the fat guy wanted to put twenty- one below the line instead of putting the one below the line and putting the two above the seven, then they added eight because seven times one is seven. All in all it is the wrong answer but it works.
Another number that works is 14 into 25. I figured this out the same way that they did in the video but with different numbers. I had to try it with a few numbers though to find 25.