A= 1+5+5 mũ 2 +5 mũ 3+…+5 mũ 99 chia hết cho 6

$\begin{array}{l}
1 + 5 + {5^2} + {5^3} + … + {5^{99}}\\
= \left( {1 + 5} \right) + {5^2}.\left( {1 + 5} \right) + {5^4}.\left( {1 + 5} \right) + … + {5^{98}}.\left( {1 + 5} \right)\\
= 6 + {5^2}.6 + {5^4}.6 + … + {5^{98}}.6\\
= 6.\left( {1 + {5^2} + {5^4} + … + {5^{98}}} \right)\,\, \vdots \,\,6\\
\Rightarrow A\,\, \vdots \,\,6
\end{array}$