Here are the answers to your questions:
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The best estimate for 18,423,850 is 20 million (since 18.4 million is closest to 20 million when rounding to the nearest million).
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10,000,000,000 rewritten as an integer power of 10 is 10^10.
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0.000001 rewritten as an integer power of 10 is 10^(-6).
If you need any further explanations or help, feel free to ask!