how 121 is correct answer?
consider the case in which sub-graph has only 2 vertices. so there are 10 ways to select 2 vertices.i’m not using set notation but you can understand.
these 10 ways to select 2 vertices are AB,AC,AD,AE,BC,BD,BE,CD,CE,DE.
out of these 10 pairs of vertices each AB,BC,CD,DE,AE will give me 2 sub-graphs. and remaining pair of vertices will give me total 5 sub-graphs.
so total i got 15 sub-graphs when i select only 2 vertices.how they are getting 20 sub-graphs with only 2 vertices?