Let $n \geq 2$ be an integer. Consider permutations $a_1,a_2,\dots,a_n$ of the set $\{1,2,\dots,n\}$
for which $a_1 < a_2$. How many such permutations are there?
(a)
$\frac{n!}{2}$
(b)
$2{n \choose 2} \cdot (n-2)!$
(c)
None of the above.
(d)
$n!$
Solution
Well, let’s break it down
We choose 2 numbers out of the n numbers: $ \binom{n}{2} $
The other n-2 numbers can be placed in any order: $ (n-2)! $