[Math] Find the number of seating arrangements at a round table of three single men, two single women, and two families

combinationspermutations

Three single men, two single women and two families take their places at a round table. Each of the two families consists of two parent and one child. Find the number of possible seating arrangements.

  1. members of the same family are seated together and the two single women are separated
  2. the seats are numbered and each child sits between their parents

My attempts,

  1. $\frac{6!}{6} + 3!\cdot2\cdot6!=87604$
    Correct Answer:$17280$
  2. $\frac{8!}{8}\cdot\ _3C_2\cdot2!=30240$
    Correct Answer:$31680$

Best Answer

For $1$, each family forms a block. There are $7!/7$ inequivalent ways to arrange the $7$ men, women and blocks. With the single women also forming a block, there are $6!/6$ inequivalent ways. In the first case, there are $3!^2$ internal permutations for the families, and in the second case there are $3!^2$ internal permutations for the families and $2$ for the single women. Thus the total number of inequivalent admissible arrangements is

$$ (3!^2)7!/7-2(3!^2)6!/6=17280\;. $$

If the seats are numbered and each child sits between their parents, there are $11\cdot7!/7$ arrangements of the men, women and family blocks, and $2^2$ internal arrangements for the two families, yielding

$$ 2^2\cdot11\cdot7!/7=31680 $$

different arrangements.