THE BEST METHOD FOR CALCULATING NO OF SK's:
total no of sk's= (# sk's over prime attributes) * (2^#no of non-prime)
here ck's are { X , Z }
# SK's over prime attributes are= { X , Z , XZ } =3 SK's over prime attributes
non-prime attributes are Y , A , B =3
total no of sk's= 3*(2^3)
=24 SK's