Cumulative Time! YAY! :( Well, it's time I begin my cumulative since I present in just a couple of weeks, and I have been stuck with the most unfortunate of topics: Consumption of Energy. The cumulative page refers me to the textbook pages to deal with "Our daily energy consumption" and "The effects of growth". Basically these two topics start by introducing me to how much energ is consumed ("despite what we may think.... WE use an enormous amount of energy every day..." blah blah blah). And then the pages lead on to talk about how fast energy consumptions grow when population growth is factored in (and how that can be a problem in this world... blah blah blah). My task is to teach this stuff to the class in approximately 20-25 minutes. The second part of this lesson is where I can see wasting a lot of that time (not "wasting" literally but, using it up... consuming it) because it deals with the whole rule of 72. You know? doubling time=72%/growth rate%/a doubling time being in years since both percentages cross eachother out. Anyways. I get the VERY basic idea of this equation: I can sub in one of the variables and determine the other. But then there is the whole task of explaining why this equation IS.. why it works, why it exists. The only resource I have for this seems to be the book, and that doesn't answer all of my questions so here I am. The other thing I need to get out of this equation is: Are there any other variables or answers I can get out of using it? The book didn't explain it but, for example, I have already figured out that by dividing 72 by the growth rate, when given a set amount of time, the answer you get is ... somehow related to the other number as to the "factor" at which the energy consumption will increase. The actual question goes like this: Q:The growth of energy consumption is 3.0%/a (annum) in LAtin America and 6.0%/a in South Asia. At these rates, by what factor will energy consumption increase after 24 years? A: 72%/3.0%/a=24 Therefore it takes 24 years for the energy consumption to double, so the energy consumption has increased by a factor of 2 in 24 years. 72%/6.0%/a= 12 Therfore it takes 12 years for the energy consumption to double, which means that after 24 years, the energy consumption will be twice doubled (increased by a facotr of 4), (24years/12years per doubling period=2) AGgH! anyways I'll stop talking now and just PRAY someone ehere knows what all of this means, and how I can spend 20-25 minutes teaching it to the class, or else just helping to explian it to me so that I am better equipped to stadn up there for taht long and speak about it. Thanks in advance!