KöMaL 2009 B4196

Let $n$ be a positive integer. What is the first digit after the decimal point in the number $\sum_{k=1}^{n} \frac{k(k+1)}{n}$ ?

Official text KöMaL. Quoted with source.

Solutions

No solution is on olymmath yet. Official and human solutions come first when they arrive. AI solutions come after them, with a label.