Right. Basic 2+2 stuff or simple solve for x is easy, but then you start deriving and integrating, working on sets with linear algebra, or going beyond simple calculus to apply it to physics, biology, and chemistry. I was at the point where even some calculus I could do in my head, but when I took quantum I had to write each step down.

Plus, if you don’t get the right answer, if you show your work the professor can show you where you went wrong so you can improve. I had a math teacher explain that in high school, and it was enough for me to take the 30 seconds to jot down the steps.