Confusion with calculation of Voltage?
Voltage is "potential difference", so, yes, if you know that point A is at +5V and point B is at +20V, then the voltage between them is -15V (the sign just says which point has the higher potential).

If you know the current trough a resistor is 2A and the resistance is 10Ohm then the voltage is 20V. This is the potential difference. If you say that one wire of the resistor is grounded (potential 0V) then the other wire will be at +20V or -20V. Or, you can say that one wire of the resistor is at +100V then the other wire will be at +120V or at 80V, depending on which wire ("upstream" or "downstream" you chose as reference).

Potentials are always referenced to some "zero" point - usually the ground.

Practical example - a PC power supply provides (among others) +5V and +12V. Between those wires is 7V potential difference and it is sometimes used to power fans if they are too loud at 12V but too slow at 5V.