It seems to be rudimentry electricty that I learned in 7th grade, some in class, some just from a book on electricty. Since V=IR and overall resistance is 720 ohms it follows that -30 Volts =i3*720 ohms or I3 = -30/720. Solve for each part of the circuit, and add the amperage to see if everything matches (within rounding error). Lets see that would have been a tough problem for 7th grade algebra. The only thing I don't remember is the power equation. I think it was I^2 * R.

To explain for those who didn't get the first part, resistance in series is addative. Resistance in parellel follows a simple rule of adding the inverse and reinverting.

Say you have a 2 ohm and a 3 ohm resistor in parellel. Most of the current will go though the 2 ohm resistor, but the effect will be a resistance that is lower than both the 2 and the 3 ohms. Take the inverse of each and you get 1/2 and 1/3 and sum them for 5/6. Reinvert the result for your answer of 6/5 or 1.2 ohms resistance.

David