Common core math on it's own isnt bad, it's the lack of fundamental understanding that leaves students behind. Take 60-38=22: common core would teach you to add 2 to 38 to get to 40 and then add 20 to 40 to get to 60 and 2+20=22. I use this math all the time as simple mental math. This doesnt teach students the traditional method of 0-8 requires the transfer of one tens place to get 10-8 for 2 in the ones place and then 5-3 for 2 in the tens place for 22. I think this lack of foundational understanding only sets up students with the math skills to make change, and not do algebra, calculus, or other advanced mathematics. I'm just speculating though and that's how I understand it.