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When an object is at a very large distance, the image is always formed at the focus of the lens or mirror. So the focal length is 20cm. Or

object distance (u)= -∞
image distance (v)= -20
focal length = f

 \frac{1}{v} - \frac{1}{u}= \frac{1}{f} \\  \\   \frac{1}{-20} - 0= \frac{1}{f} \\  \\ f=-20cm

So focal length is 20cm
3 5 3
ok now?
1/u is 0 as u is infinity
Thanks Alizina!!