The end points of the different admittances which are connected directly to the common point B have been marked as 1, 2 and 3 as shown in the Fig.
Incidentally, 40 Ω resistance will not be taken into consideration because it is not directly connected to the common point B. Here V01 = VA1 = −50 V ; V02 = VA2 = 100 V ; V03 = VA3 = 0 V.
∴ V100' = VAb = ((-50/50) + (100/20) + (0/10))/((1/50) + (1/20) + (1/10)) = 23.5V
Since the answer comes out to be positive, it means that point A is at a higher potential as compared to point B.
The detailed reason for not taking any notice of 40 Ω resistance are given in