As the image is obtained on the screen , it is real .

so,

Magnification , m = –3 ,

v = 80 cm

u = ?

As m = v/u

so, –3 = 80/u,

u = –80/3 cm .

From

1/f = 1/v - 1/u

=1/80 + 3/80

= 4/80

= 1/20

1/f = 1/20cm

so, **f = 20 cm . **

The lens is convex and image formed at 80 cm from the lens is real and inverted.