For the following object distances and focal lengths of thin lenses in air, find the image distance and the magnification and state whether the image is real or virtual and erect or inverted:

(a) s = 40 cm, f = 20 cm;

(b) s = 10 cm, f = 20 cm;

(c) s = 40 cm, f = –30 cm;

(d) s = 10 cm, f = –30 cm.