Total no. of combinations are 4*3*2*1=24
There are 6 ways for making a pair of B-a, similarly 6 ways are possible for the pair A-c in these ways 2 ways are common hence total no. of rejected ways
=6+6-2=10
Similarly 6-6 ways are possible for the combination of C-b and C-d, in these ways 3-3 ways are affiliated with pair A-c and B-a hence only 3-3 ways are acceptable
A representation for the pair C-b is
C-b A-a B-c D-d (selected)
C-b A-a B-d D-c (selected
C-b A-c B-a D-d (rejected)
C-b A-c B-d D-a (rejected)
C-b A-d B-a D-c (rejected)
C-b A-d B-c D-a (selected)