Today at work I was thinking hypothetically about a simple circuit that I was unable to get an answer that made any sense. What if you had a simple 12V source with a 5Ω resistor in series, also in series with a black box that consumes 60W of power. You aren't given any characteristics about the black box other than it dissipates 60W. When I do a KVL around the simple loop, I obtained 12V - 5i - 60/i = 0, since P = VI and for this device V and I are both unknown. If I multiply through by i, you can obtain a quadratic formula, but neither of the results physically make sense; in this example they result in complex numbers, and even the magnitude of the real part isn't near what I was expecting. This was driving me nuts today and it is so simple, but somewhere I am not right. Any ideas?