.
1. You can choose seven of 15 by ways to the first, 7-seating wheel.
= .
2. You can choose five of remaining 15-7 = 8 by ways to the second, 5-seating wheel.
= .
3. And that is all. The remaining 15-7-5 = 3 persons are just uniquely defined, and you have nothing more to select.
4. So, your final answer is the product of these two numbers:
.
5. I leave it to you to complete these calculation.
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On Combinations, see the lessons
- Introduction to Combinations
- PROOF of the formula on the number of Combinations
- Problems on Combinations
- OVERVIEW of lessons on Permutations and Combinations
in this site.
Also, you have this free of charge online textbook in ALGEBRA-II in this site
- ALGEBRA-II - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this online textbook under the topic "Combinatorics: Combinations and permutations".
Save the link to this textbook together with its description
Free of charge online textbook in ALGEBRA-II
https://www.algebra.com/algebra/homework/complex/ALGEBRA-II-YOUR-ONLINE-TEXTBOOK.lesson
into your archive and use when it is needed.