The method of subtraction in the video is not required by common core. The method of subtraction in the video predates common core. Common core doesn't mandate that only one method of subtraction be taught. Common core encourages teaching several different methods of subtraction and making sure that students know when to use which one. You can read more here: corestandards.org There are valid concerns with common core, particularly the K-3 standards, but, unfortunately, very little of what gets brought up when discussing common core is actually related to common core. For example, this entire thread.