Gosh I detest the math book that we are using in calculus. It doesn’t show you any of the algebraic thought process, just a bunch of high-level descriptions. You are essentially forced to figure out how to do it by trial and error. Well at least I derived the half-life formula finally:

percentage of amount remaining = 100(.5)^(total years/years of half-life)

I expect better out of a text book especially if it is costs around $150.