The circumference of Earth is estimated to be 40,000 kilometers at the equator. Which of the following best approximates the diameter, in miles, of Earth's equator?

(1 kilometer \(\approx\) 0.62137 miles)

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Find the diameter first using the circumference formula:

$$ C=2\pi r $$ $$ C=\pi d $$ $$ 40{,}000=\pi d $$ $$ d \approx 12{,}732 \text{ kilometers}$$

Convert to miles.

$$ 12{,}732 \cancel{\text{ km} } \cdot \frac{0.62137 \text{ mi}}{1 \cancel{\text{ km}}}$$ $$ \approx \boxed{7{,}912 \text{ mi}} $$