A person looks at her image in a shiny spherical decoration when
her face is 30 cm from the surface of the decoration. The diameter
of the decoration is 20 cm. Find the position of the image
The spherical decoration is the equivalent of a convex mirror - which means the centre point (C) will be half the diameter (10cm). The focal point is always half the distance between the centre and the surface, so F = 5cm. As long as you remember that the image distance and focal point are negative values, it should work out like a regular optics question.