An air traffic control towel at an airport located in flat d

An air traffic control towel at an airport located in flat dessert terrain, the controller can see to the horizon. The distance d in miles that he can see from a height above ground level h (in feet) is given by the following equation. How high must the controller be to see 20 miles to the horizon? Round to the nearest foot.

D=(the sq root of 1.5h

Solution

How high must the controller be to see 20 miles to the horizon? Round to the nearest foot.
D=(the sq root of 1.5h
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20 = sqrt(1.5h)
Square both sides to get:
1.5h = 400
h = 266 2/3 ft.