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In how many ways can $9$ friends sit at a circular table if two of them, Anna and Bob, insist on having exactly two people seated between them? (As usual, two seatings are considered the same if one is a rotation of the other.)

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There are 10,080 different ways in which the friends can seat on the circular table.

In how many ways the friends can seat?

There are 9 friends, such that two of them need to be separated by exactly two people.

Because the table is circular, we can consider the first position as the position where Bob is.

Now let's count the number of options for each of the other 8 positions. (counting to the left).

The next two positions have 7 and 6 options respectively (as these can be taken by any of the other 7 friends)

For the next seat, we could seat Anna or one of the remaining 5 friends.

Let's assume we seat Anna there, then for each of the next positions, we will have, respectively, 5, 4, 3, 2, 1 options.

The total number of combinations is given by the product between the numbers of options, so we have:

C = 7*6*5*4*3*2*1

But we also need to consider the case where Anna is on the first position (and Bob on the third), so we just need to add a factor equal to 2.

C = 2*(7*6*5*4*3*2*1) = 10,080

There are 10,080 different ways in which the friends can seat on the circular table.

If you want to learn more about combinations:

https://brainly.com/question/11732255

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