There are ten cards on the table, and numbers 1 to 10 are written on them.
I will put them into 5
identical
boxes such that every box will contain two cards.
The two numbers in each box will be coprime numbers.
How many ways can I do this?
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Since there are five even-numbered cards and five boxes, each box must get one and only one even-numbered card. Let's suppose this has been done; then the other five (odd-numbered) cards can be distributed among the five boxes in 5 ! ways. However, neither card 3 or card 9 can go into the same box as card 6; also card 5 cannot go into the same box as card 10. To eliminate these possibilities, we can use inclusion-exclusion: we subtract from 5 ! the number of ways of distributing the cards so either card 3 or card 9 does go into the same box as card 6, as well as the number of ways that card 5 goes into the same box as card 10; then we must add back in the number of ways of both of these happening.
The number of ways of distributing the five odd-numbered cards so that 3 or 9 go into the box containing 6 is 2 × 4 ! .
The number of ways of distributing the five odd-numbered cards so that 5 goes into the box containing 10 is 1 × 4 ! .
The number of ways of distributing the five odd-numbered cards so that 3 or 9 go into the box containing 6 AND 5 goes into the box containing 10 is 2 × 1 × 3 ! .
Thus the number of ways of distributing the five odd-numbered cards so that each box contains a coprime pair is
5 ! − ( 2 × 4 ! ) − ( 1 × 4 ! ) + ( 2 × 1 × 3 ! ) = 1 2 0 − 4 8 − 2 4 + 1 2 = 6 0 .