A cup of 12 cm high is placed in front camera obscura at 2 m from the hole. The distance from the hole to the opposite wall is 30 cm. Calculate the height of the image projected on the wall.

A lamp of negligible dimensions is fixed on the ceiling of a room, whose height is 3 m. An opaque disc of 20 cm in diameter is suspended at 75 cm from the ceiling, their faces are horizontal and their center is in the same vertical as the lamp. Calculate the shadow area projected by the disc above the floor.

On a sunny day, a building projects a shadow of 10 m in length at the same instant, a 5 m high pole projects a shadow of 2 m in length. Determine the height of the building.