<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] $5 + 4.5 + 4.5^2+4.5^3+4.5^4+4.5^5 = 5+5(5-1)(1+5+5^2+5^3+5^4) = 5+5(5^5-1) = 5^6.$
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