Since $10^x - 10 = 9990,$ we have $$10^x = 9990+10=10000.$$If $10^x = 10000,$ then $x=\boxed{4},$ since $10000$ ends in four zeroes.
Question:
If $10^x - 10 = 9990,$ what is $x$ equal to?
Answer:
Let's think step by step.

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4
If $10^x - 10 = 9990,$ what is $x$ equal to?
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