Six persons A,B,C,D,E and F are to be seated at a circular table. The number of ways this can be done is A must have either B or C on his right and B must have either C or D on his right is_______.

When A has B or C to his right we have  AB or AC
When B has C or D to his right we have BC or BD
Thus we must have ABC or ABD or AC and BD
For ABC,D,E,F on a circle
no. of ways =3!=6
for ABD,C,E,F on a circle
no. of ways = 3!=6
for AC,BD,E,F the no. of ways = 3!=6
Total =18