Ohm’s Law and More

I was reading my general exam manual, chapter 5, electrical principles, and was being amazed by what I was learning for the second time. I got ahead of myself when reading about parallel circuits, specifically resistors. The first part focused on current, but I was confused, ohms law says that a resistor on 100 volts at 1 amp should be 100 ohms. The diagram I was looking at had two 400 ohm resistors and one 200 ohm resistor in parallel. How did this become 100 ohms? If I would have just read on a few pages, I would have figured it out, but I guess I’m just impatient. How does 400, 200, & 400 become 100? I filled up an 8.5 X 14 sheet of paper figuring it out. Basically, it is 1/(1/400 + 1/200 + 1/400) or 1/(4/400) or 1/0.01 = 100! I’m not a math in my head whiz, it’s a good thing my blackberry has a calculator, I was determined to figure it out. Of course, my theory was confirmed a few pages in the future. I also figured out the section about power (watts), but that was easy, I remembered the basic concept, the book just helped me remember how to associate figuring power with the help of ohm’s law. Fun!