If we arbitrarily set VA at 0 and then find VB, VC, VD, we could represent them as a "voltage ladder". From the ladder it is easy to see:
VCA = 8 + 5 = 13 V
VDA = 12 + 8 + 5 = 25 V
Therefore VAD = -25 V
VDB = 12 + 8 = 20 V

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