The formula for the area of a triangle is 1/2 * base * height.
Since the area must be under 20 ft^2, we have:
1/2 * base * height < 20
Since the base is 20 feet, we have:
1/2 * 20 * height < 20
10 * height < 20
height < 20 / 10
height < 2
Hence, the height must be less than 2 feet.