A concave mirror produces a 20cm long inverted image of an object of height 4cm. What is the magnification of the image?

 Height of image(I)= -20 cm (Inverted image is denoted by negative sign)

Height of object(O) = 4 cm 

Magnification is given by the ratio of the height of the image to the height of the object.

Magnification= $\frac{\mathrm{I}}{\mathrm{O}}= \frac{-20}{4}= -5$

Therefore, the magnification of the image will be -5. The negative sign indicates that the image is real and inverted.