the earth is a spherical object, and you're correct, its radius is 3959 miles.
we know 71% of that is covered in water, that means 29% of that is not, 71%+29% = 100%.
well, let's recall that whatever% of anything is just (whatever/100) * anything.
now, how many square miles is its surface anyway?
[tex]\bf \textit{surface area of a sphere}\\\\
SA=4\pi r^2~~
\begin{cases}
r=radius\\
-----\\
r=3959
\end{cases}\implies SA=4\pi (3959)^2
\\\\\\
SA=62694724\pi [/tex]
since 71% is already covered with water, we only need the other 29% covered, what is 29% of 62694724π, well, is just (29/100) * 62694724π.
