I'm torn on some of this stuff. I'm 37. I could have done the math on that 15 years ago. But I couldn't do it free hand today. Besides, there's an app for that 😛
Reality is so many of us are taught how to do things, and then taught the "easy" way. Once you have shortcuts you forget the hard way to do things. I've got so much other "stuff" to know and remember somethings just get shoved out of the way. I'm sure an accountant, actuary or somebody that has to work with numbers on a daily basis can solve that interest problem. But I don't work with it daily. Haven't worked with it in a couple decades and frankly, can google the answer faster than doing it myself.
In it's place I now have 300 different server names and their functions, can ramble off HL7 interface specs, can navigate without directions around multiple major US cities, and break down a DICOM conformance statement. You just tend to purge the shit you aren't using any more.