physics
posted by steph .
A lightbulb emits light that creates a shadow of a ball on a wall. If the radius of the ball is 12.3 cm, and the radius of the shadow is 29.5 cm, and the ball is 1.27 m away from the wall, how far is the lightbulb away from the wall? (For the present problem, you may assume that the light bulb is a"point source".)

view the setup from the side, with the line joining the light source and the center of the ball perpendicular to the wall.
Then, if the light is x cm away from the wall, using similar triangles,
x/29.5 = (x127)/12.3
x = 217.8 cm