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.