Question
Let $S_n$ be the sum to $n$-terms of an arithmetic progression $3,7,11$, If $40<\left(\frac{6}{n(n+1)} \sum_\limits{k=1}^n S_k\right)<42$, then $n$ equals ________.
Correct Answer
9
Let $S_n$ be the sum to $n$-terms of an arithmetic progression $3,7,11$, If $40<\left(\frac{6}{n(n+1)} \sum_\limits{k=1}^n S_k\right)<42$, then $n$ equals ________.