15. For an arithmetic sequence $\left\{ a _ { n } \right\}$ with sum of first $n$ terms $S _ { n }$, if $a _ { 3 } = \frac { 3 } { 2 } , S _ { 4 } = 10$, then $\sum_{k=1}^{n} \frac { 1 } { S _ { k } } = $ ______ [Figure] III. Solving Problems: Questions 16-23 [Figure][Figure]
15. For an arithmetic sequence $\left\{ a _ { n } \right\}$ with sum of first $n$ terms $S _ { n }$, if $a _ { 3 } = \frac { 3 } { 2 } , S _ { 4 } = 10$, then $\sum_{k=1}^{n} \frac { 1 } { S _ { k } } = $ ______\\
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III. Solving Problems: Questions 16-23\\
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