1. The problem statement, all variables and given/known data Two solenoids on the same cylindrical axis Solenoid A has 400 loops and a current of 3,5 amp. Produces a flux of 300microWb on itself and 90 microWb on solenoid B Solenoid B has 700 loops 2. Relevant equations Calculate the energy stored in the system. 3. The attempt at a solution Well, I know that the energy stored in magnetic field equals the square of the field over two times mu zero, but that's not the case here since I don't have the field. I also know that for a self-inductor the energy stored equals to the square of the current times the self induction over two. I believe I need to use that last equation, but as I don't understand where it comes from I don't know what to put as self-induction, or if it works also when the induction is produced by another coil.