Qual é o algarismo das dezenas da soma? $$\underset{\begin{array}{c}um\\ sete\end{array}}{\underbrace{7}}+\underset{\begin{array}{c}dois\\ setes\end{array}}{\underbrace{77}}+\underset{\begin{array}{c}tr\hat{e}s\\ setes\end{array}}{\underbrace{777}}+\underset{\begin{array}{c}quatro\\ setes\end{array}}{\underbrace{7777}}+...+\underset{\begin{array}{c}setentaeseis\\ setes\end{array}}{\underbrace{\mathrm{777...77}}}+\underset{\begin{array}{c}setentaesete\\ setes\end{array}}{\underbrace{\mathrm{777...777}}}?$$