X, Y and Z are three permeable ions. If at resting potential, X and Y are - 50 and - 30 respectively. What is the value of Z?

Now in this question: The ions are freely permeable their conductance can be considered as same or simply 1. There is no mention of impermeant ion (negative impermeant protein along with selective permeability of cell membrane is responsible for negative RMP in cells), hence at rest or equilibrium, an electrical neutrality will be established resulting in the potential across membrane to become zero. Thus, Ex + Ey + Ez = 0; i.e; Ez = +80mV