Easy or difficult?

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I'm finding the course I mentioned a few days ago, Plate Tectonics, to be understandable, hard but interesting. Until, that is, they ask me to calculate something like the speed at which one plate is moving relative to another, assuming the other as stationary, assuming a different plate as stationary... or how fast has it moved calculated in x to the whatever power of years by mm per year. I seriously have no idea. I never was good at remembering or understanding mathematical formulas, and the last time I had to use them was in 1968 or so when I took algebra, and then all I had to do was plug in numbers. I could do that without understanding the formula itself. So, I'm gaining a lot of interesting information from this course, and I'm good at deducing probabilities from facts given, but I am totally clueless when it comes to calculating anything! What makes things easy or difficult for you when doing an activity in real life or a course in education?