Element B in the diagram supplies 60 W of power. Calculate I_x.
Element B in our question supplies power which means power should be leaving out of the positive terminal. However, in the diagram, we see that the current is travelling in the opposite, and is entering the positive terminal, which means our answer will be negative.
p=vi where p is power, v is voltage and i is current.
Substituting the values and isolating for i gives us:
This question can be found in Basic Engineering Circuit Analysis, 10th edition, chapter 1, question 1.24.