Consider all permutations of integers $1,2,3.....100$. In how many of these permutations will the $25^{th} $ number be the minimum of the first 25 numbers and the $50^{th} $ number be the minimum of the first 50 numbers?
My attempt:
Total number of permutations: $100!$
The first 25 numbers can arranged in this manner:
$100,99,98......76,75\Rightarrow \text{the total number of permutations}=24!$
The arrangement of the next 25 numbers:
$74,73,72..............51,50\Rightarrow \text{permutations possible} =24! $
The last 50 numbers can be arranged in $50!$ ways
$\therefore \text{the sequence becomes}: 100,99,98, .....\color{red}{75},74,73, .....\color{red}{50},49,48,......1 $
The total permutations of such sequences are : $ 24!\times24!\times 50! \times 2$
$[\times{ 2}\text{ because the numbers can also be arranged in 50,49,48...25,24...1,51,52...100}]$
I am sure that the answer is not correct. Any ideas?
Where did you get this question from?
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