To create super keys we can combine the candidate key with the sets formed with the remaining attributes.
So the number of superkeys we can form with candidate key {A}$= A = 2^{n-1} = 2^{5-1} = 16$
Number of superkeys we can form with candidate key {BC} $= B = 2^{n-2} (here n=5) = 2^{5-2} = 8$
Now there are several key sets that we have counted twice when calculating both separately, like {A,B.C} , {B,C,A} & {A,B.C,D} , {B.C,D,A} etc.
So as per set theory
If A ∩ B ≠ φ, then
n (A ∪ B) = n (A) + n (B) – n (A ∩ B)
$superkeys = 16 + 8 - 2^{5-3} = 24-4 = 20$
Hence 20 super-keys are possible.