Make a vertical line from point D to OB, intersect with OB at point E, and the vertical foot is E. According to the similarity of right triangle, we can get that the triangle DEB is similar to the triangle CFD (let CF be high H and F be vertical foot), so BD: CD = DE: H.

OB=60cm, AD=30cm, BE=30cm, DE=OA=30cm, so BD=50cm, so h=4m is obtained.