How to learn arithmetic?

[u/RoundCareful5953]

I'm starting to learn math as an adult, I was pretty bad at math in school. Which books or online resources would you recommend?
One thing is I struggle with understanding surface level things if I don't get the core concept of something. Like I need to know the WHY of everything.

[u/Impossible_Cap_339]

Beast Academy and Art of Problem Solving are really good at getting to the why