Final answer:
The sum of the concentration of ions determines the direction of their movement across a membrane.
Step-by-step explanation:
In the presence of two or more ions, it is the sum of their concentration that determines the direction of movement of those ions across a membrane. This is because the movement of ions across a membrane is driven by both their concentration gradient and the electrical force exerted by the membrane potential. Together, these forces create an electrochemical gradient that influences the movement of ions.