I have the following in my book:

"$\displaystyle (N K ) = n! / k! (n - k)!$

This formula is symmetric in k and n-k:

$\displaystyle ( N K ) = ( n / n - k)$"

I'm trying to understand what it means with 'this formula is symmetric in n and n-k'. I have tried to look for an answer, but haven't found anything. Does anyone know what that is supposed to mean?

Can you explain it please?