Solve the Problem $d$ (In Miles) That an Observer Can See on a Clear

Solve the problem.
-The distance $d$ (in miles) that an observer can see on a clear day is approximated by $d = \frac { 49 } { 40 } \sqrt { h }$ , where $h$ is the height of the observer in feet. It Rita can see $24.5 \mathrm { mi }$ , how far above ground is her eye level?

A) $400 \mathrm { ft }$
B) $40 \mathrm { ft }$
C) $20 \mathrm { ft }$
D) $6 \mathrm { ft }$