# 4 and 5

For some positive integer $$k,$$ both $$4^k$$ and $$5^k$$ start with the same digit $$x$$ in base 10: \begin{align} 4^k &= \overline{x\dots} \\\\ 5^k &= \overline{x\dots}.\end{align} What is the sum of all possible values of $$x?$$

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