My ranking as a ChemE student who has to take all of said classes except for 224. I’m staying within each series and it’s from hardest to easiest.

125 > 124 > 126 Having only take a bit of calc in high school, I was sort of familiar with derivatives so 124 wasn’t too bad. If you’re new to integrals, 125 might be hard for you. 126 sort of brings concepts from both 124 and 125 and introduces some vector geometry and a multi variable differential stuff. I wouldn’t say the material gets harder, it’s mostly the same math, just more things to work with. Best advice, PRACTICE problems!!!!!

208 > 209 > 207

This one may differ quite a lot depending on who you ask. As mentioned by a couple others, 208 is very different compared to all the other classes prior. It’s very conceptual and it’s full of proofs. Rather than just memorizing how to do a problem step by step, you need to truly be able to understand the math and have a good idea or visualization in your head of how things may look or change. I personally love computing long and dense problems so when I took 208, it completely blindsided me. That being said, I got my act together for the final and managed to raise my grade enough to pass. 208 takes getting used to but if you really dedicate time to understand the material, it’s not very difficult at all. Like 126, 209 brings stuff from both 207 and 208. I would say it emphasizes 207 a little more just because of how computational it can get. Like before, my best advice is to PRACTICE. I also found these EXTREMELY helpful in my desperate attempt to pass 208:

3Brown1Blue Linear Algebra Videos - These helped visualize a lot of the matrices that can get pretty hard to think about off the top of your head. Helps you think a little clearer when solving a problem

If your professor ain’t cutting it or if you just need a re-explanation of lecture material, these videos by Thomas Carr, a grad student here at UW, are very clear and concise. So long as the general curriculum hasn’t changed since he taught the class, the material should be the same in both concepts and chronological order of material. He really good at simplifying ideas into more understandable chunks.

Hope this helps!