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In how many ways can 15 people be seated around two round tables with seating capacities of 7 and 8 people?

Solution

✅ Correct Option: 4

When people sit around a round table, rotations look the same. If everyone shifts one seat clockwise, it's still the same arrangement. For nn people at a round table, there are (n−1)!(n-1)! arrangements (not n!n!).


First, decide which people sit at which table: 7 people go to the 7-seater table and 8 people go to the 8-seater table.

Number of ways to choose 7 people from 15:

15!7!×8!\dfrac{15!}{7! \times 8!}


For the round table with 7 people, the number of arrangements is:

(7−1)!=6!(7-1)! = 6!


For the round table with 8 people, the number of arrangements is:

(8−1)!=7!(8-1)! = 7!


Total number of ways:

=15!7!×8!×6!×7!= \dfrac{15!}{7! \times 8!} \times 6! \times 7!

=15!×6!×7!7!×8!= \dfrac{15! \times 6! \times 7!}{7! \times 8!}

=15!×6!8!= \dfrac{15! \times 6!}{8!}

=15!8!×6!= \dfrac{15!}{8!} \times 6!

Therefore, the answer is 15!8!×6!\dfrac{15!}{8!} \times 6!

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