TO solve this problem, we simply need to use ratio and proportion. From the statement problem we can say the ratio height : distance, so that: (let us say that X is the image distance)
15 cm : 30 cm = 10 cm : X
We know that the symbol : means division so:
15 cm / 30 cm = 10 cm / X
X = 10 cm * 30 cm / 15 cm
X = 20 cm
So the image distance is 20 cm
