There are 9 bottle labelled 1, 2, 3, ... , 9 and 9 boxes labelled 1, 2, 3,....9. The number of ways one can put these bottles in the boxes so that each box gets one bottle and exactly 5 bottles go in their corresponding numbered boxes is
🎥 Video solution / Text Solution of this question is given below:
Total bottles and boxes: 9 each, labeled 1 to 9.
We are asked to count permutations of bottles such that exactly 5 bottles go into their own numbered boxes.
Step 1: Choose 5 positions to be fixed points (i.e., bottle number matches box number).
Number of ways = $\binom{9}{5}$
Step 2: Remaining 4 positions must be a derangement (no bottle goes into its matching box).
Let $D_4$ be the number of derangements of 4 items.