The standard medical unit for potassium (and many other chemical substances in the body) is "milliequivalents per liter" or mEq/L. Note that this is a concentration, not a quantity. If we assume that the volume of the bloodstream is constant (which is approximately true), then the mEq/L units are as good as units that actually measure how much potassium is present. The units in this problem are consistent, so we will not worry too much about what they mean exactly.