<s>[INST] Evaluate $5+4\cdot 5+4\cdot5^2+4\cdot5^3+4\cdot5^4+4\cdot5^5$ 
Evaluate
  $$5+4\cdot 5+4\cdot5^2+4\cdot5^3+4\cdot5^4+4\cdot5^5.$$
  The options are $5^6$, $5^7$, $5^8$, $5^9$, $5^{10}$.

I'm new to this site. I came across this question in an Olympiad foundation site. I have no idea how to solve it. Can I get the answer of this question. Thanks.
 [/INST] Hint. Note that the given sum can be written as
$$5+(5-1)\cdot 5+(5-1)\cdot5^2+(5-1)\cdot5^3+(5-1)\cdot5^4+(5-1)\cdot5^5.$$
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