A boy whose eyes are 1.5 m above the ground stands 10√3 m from a vertical pole and finds the angle of elevation of the top of the pole to be 30°. The height of the pole is:

The angle is measured from the boy's eye level, so the top of the pole stands 10√3·tan30° = 10√3/√3 = 10 m above his eyes. His eyes are already 1.5 m up, so the pole is 10 + 1.5 = 11.5 m tall. Forgetting to add the eye level leaves 10 m, and subtracting it gives 8.5 m.