This should be a question of permutation and combination.

C 10/2

Combinatorial problem

Choose two companies from nine companies to sign contracts with each other.

There are 45 kinds of C10/2 * * * written in the combination formula.

If you haven't learned permutation and combination

I think we can fight.

Every two companies sign the company number 1 2 3 4 5 6 7 8 9 10.

1 Sign contracts with 9 other companies.

Sign with the remaining eight people.

Three of them sign with the remaining seven.

……

8 and the remaining 1.

So it's equivalent to one to nine.

***45