When I was an undergrad working on my teaching certificate, I had a wonderful math methodologies professor who taught us something very important the first day of the first math methodologies class.
The most important math skills you can teach a child are place value and ratios. A great deal of math builds on one or both of these foundational skills.
It took my becoming a math tutor to understand what he was trying to tell us, but it’s slowly sinking in. It’s amazing how often, if a student is having difficulties, we back up to either doing a refresher of place value or ratio skills. Dr. Drum had it right. So many math skills become simpler if you have a good handle on both place value and ratios!