When people sit around a round table, rotations look the same. If everyone shifts one seat clockwise, it's still the same arrangement. For n people at a round table, there are (n−1)! arrangements (not 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:
7!×8!15!
For the round table with 7 people, the number of arrangements is:
(7−1)!=6!
For the round table with 8 people, the number of arrangements is:
(8−1)!=7!
Total number of ways:
=7!×8!15!×6!×7!
=7!×8!15!×6!×7!
=8!15!×6!
=8!15!×6!
Therefore, the answer is 8!15!×6!