The answer is 0.20 km
The function is D(h) = 111.7 * √h,
where:
D(h) is the maximum distance that can be seen from a height h.
We have:
D(h) = 50 km
h = ?
D(h) = 111.7 * √h
50 = 111.7 * √h
50 / 111.7 = √h
0.45 = √h
Now square both sides of the equation:
0.45² = (√h)²
0.2025 = h
h ≈ 0.20 km