M → \(\frac{Hi}{Ho}\) = \(\frac{Di}{Do}\)
where M is the magnification, Hi is the height of the image, Ho is the height of the object, Di is the distance from the lens to the in-focus projected image, and Do is the distance from the object to the lens.
But Di = \(\frac{Hi * Do}{Ho}\) → \(\frac{20 * 30}{40}\)
Image distance(Di) = 15cm