29°
Consider the right triangle between the plank and the ground
the angle can be found using tangent ratio
tanx° = [tex]\frac{opposite}{adjacent}[/tex]
where adjacent = 18 and opposite = 10
tanx° = [tex]\frac{10}{18}[/tex]
x = [tex]tan^{-1}[/tex]([tex]\frac{10}{18}[/tex]) = 29.0546 ≈ 29°
