Answer
7
Work Step by Step
$S_{n}=\dfrac {a_{1}+a_{n}}{2}\times n\Rightarrow a_{1}=\dfrac {2S_{n}}{n}-a_{n}\Rightarrow a_{1}=\dfrac {2\times S_{20}}{20}-a_{20}=\dfrac {2\times 1090}{20}-102=7$
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