Respuesta :

Explanation:

Here,

object distance(u)=20cm

image distance(v)=80cm-20cm=60cm

focal length(f)=?

we know ,

(1/f)=(1/v)+(1/u)

1/f = 1/60 + 1/20

1/f = (1+3)/60

60 =4f

f=15cm