Set up KVL in a simple circuit
The open circuit can be replaced by a resistor with infinite value (R3)



If we assume that R3 indeed is infinite, R1 and R2 become so small in relation to R3, that we can disregard them. U = I*R, and since current is constant in a series connection, resistance will dictate the voltage distribution across resistors. Hence all voltage will fall across the infinite resistor R3. Now if we agree that R1 and R2 are so small we can disregard them, we are left with this circuit


Now we just add all sources. VR3 = V1-V2 = 10 - (-10) = 10+10 = 20V

Hope this helps.