Answer
b. 8 hours
Work Step by Step
We use the equation for period to find:
$T=\sqrt{\frac{4\pi^2r^3}{GM_E}}$
$T=\sqrt{\frac{4\pi^2(20,200\times10^3)^3}{(6.67\times10^{-11})(5.97\times10^{24})}}\approx \fbox{8 hours}$
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