D1 Question - Order of an algorithm

Here's the question:
'Explain why the first-fit bin packing algorithm has a quadratic order.'

The online answers page says this:


So in other words, we're going from k items requires k-1 comparisons to plucking a magical formula out of thin air without any explanation.

k items, I get that. k-1, I get where they are coming from. 1/2n(n-1), I do not get that AT ALL.

What I want is a METHOD to how they've gotten from k and k-1 to n(n-1)/2. Please someone, fill in the gaps. Thankyou.