Answer
$57,000$ ft
Work Step by Step
We want $r$ for which $D=0$. We take the result of (b) and equate D with $0$.
$D=95-20\log r$
$0=95-20\log r$
$20\log r=95$
$\log r=95/20$
$r=10^{95/20}\approx 56234.13$
Rounded to $2$ significant figures, the result must be more than $56,000$ ft, so our answer is $57,000$ ft.
