BdMO Online Forum

$6$ people sit down around a circular table. This group of $6$ people includes a pair of identical twins . How many ways can they be seated? ?

note: $2$ seatings are considered the same if one can be rotated to form the other. Two seatings are also considered same if the two twins switch positions.

answer is $5!/2!$

if you want to know how it works , read any books of combinatorics .
it could be "principles and techniques in combinatorics" or google it