In the videos, the people had a hard time keeping numbers straight. For example, when the two different men divided, they both divided the number in the ones place first. They should have divided the whole number instead. Then, they brought down the number in the tens place, which means they did the long division backwards. When you multiply, you are supposed to put a number at the end of the second product before you add both of them together. The people who multiplied in the videos did not do that, and that made the 210 a 21 and the 50 a 5. Last, but not least, when the people added, they did not count the tens in groups of ten. The tens became ones that were added into the ones place. I figured it out by comparing the videos to what I normally did in math. Every little number counts.