 He said that they only have to memorize four and then you can drive the rest from the four by understanding the concepts. Yeah, but the problem is when you're writing a final exam, you don't have time to drive all the formulas, right? That's why I said that we probably memorized around 20 of them, right? And from that 20, you would be skipping certain steps, right? So for example, and I did this by the way for a geophysics course that I took. So let's assume, so think about as modules, right? Let's assume you're given a question that requires this formula here, right? You need to know this formula here, right? But you haven't memorized this formula. Your base formulas would be the ones that you would know. One, two, three, four. So you can memorize those formulas or know those formulas, right? And with these formulas, you can do combinations, mix and match, do whatever you need to do to drive all the other formulas, right? However, however, it might take you a long time to get to this one from just the four. But what you can do, you can do cheats, right? You can memorize an additional 16 formulas or 10 formulas or whatever it is to skip certain steps, right? So for example, let's say to get to here, you would have to hit this one and you hit this one. But you can't go from these to these directly to this. But if you remember to this, remember this one, you've skipped all these steps here, right? And if you need that one as well, then you've skipped all those steps here. And if you need to get this from this guy and this guy, then you can go there, right? Or you need this guy, this guy and these two, then you could go there from this. Or you need this guy and this guy and one of the other ones you haven't memorized, then you can take these four formulas, derive the one that you need as well to get to that one, and then use this, this and this to get to that one, right? So what you need to do if you're taking this course that requires 100 formulas is learn the original four and learn key formulas in the process of getting to some of the more complicated formulas. Okay.