Differential equations. It was very tough, and I am quite good at math, though I find it to be painfully boring. I was never so happy to get a B in a class.
For those unfamiliar with differential equations, or “diffy-q’s,” as we students often called it, it is an extension of calculus.
It involves enormous equations in which each term is like a mini calculus problem. Half of the battle was separation of variables, getting all the Y’s on one side of the equation, and all of the X’s on the other side.
I was a chemical engineering major my first two years of undergrad.