One of those things you have to learn when you're in the Audio industry (and many others) is the Ohm's Law. Yes, that annoying pie chart that looks like a jumble of letters and math equations:
It's pretty simple really, to find the Wattage, Current, or Voltage of something, just plug in the numbers you know and you'll get your amount. Where might this be useful to you? Think of a conventional household 15 Amp Circuit breaker. How can you avoid tripping the breaker? Using the Ohm's Law! Lets calculate the max amount of power draw you can use on this breaker before you trip it. Using the Ohm's Law all we need to do is calculate the amperes times the voltage (120v is the standard household AC voltage) 15x120 and our result is 1800 Watts. So this means we can use 30 60 watt incandescent lightbulbs until we trip that circuit breaker.
The Ohm's Law also is vital in the lighting business which I dabble in and also in Amateur Radio (often called Ham Radio) which is what is peaking my interest right now. The FCC Technicians Class License exam will have some questions on it requiring you to calculate power using the Ohm's Law. Sure the charts are handy, but here's how you can memorize the formulas without them:
All you have to remember is the Indian, the Rabbit, and the Eagle:
I= Indian (or Amperes)
R= Rabbit (or Resistance)
E= Eagle (or Voltage)
How does this work? Well when the Eagle is flying, it sees the Indian and Rabbit on the same level so E=IxR. When the Indian is looking out it sees the Eagle above the Rabbit so I = E/R an when the Rabbit is looking across the horizon it sees the Eagle over the Indian so Resistance equals Voltage divided by Amperes.
There you go! This is how I remember the Ohm's law and I've never forgotten it since! Yes you can get an app or use an online calculator, but it's way more cool to know it off the top of your head :)