Chapter 12 - Sequences; Induction; the Binomial Theorem - 12.1 Sequences - 12.1 Assess Your Understanding - Page 807: 66

$\sum_{k=1}^{n}(\frac{k}{e^k})$

We can see that this is the sum of the $n$ fractions in which the first denominator is $e$, and gets multiplied by $e$ in each term and in which the numerator of the nth term is $n$. Hence the summation formula is: $\sum_{k=1}^{n}(\frac{k}{e^k})$