Algebraic Topology Foundation (Lecture 1 to 6). Very easy and helpful to undergrads!
Lecture 3: Modular Arithmetic (easy)https://youtu.be/5YEu-vB2dD8
Lecture 4: Addition and Free Vector Spaceshttps://youtu.be/9qL003DG7Og
Free Abelian Groups & Free Vector Spaces
Mathtuition88 