I want to get the solution for this

Dear Student,
Answer to your question is given below:

G i v e n, f o c a l l e n g t h, f = 20 c m m a g n i f i c a t i o n = 4 \Rightarrow \frac{v}{u} = 4 \Rightarrow v = 4 u N o w, w e k n o w t h a t, \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \Rightarrow \frac{1}{- 20} = \frac{1}{4 u} + \frac{1}{u} \Rightarrow \frac{1 + 4}{4 u} = \frac{1}{- 20} \Rightarrow u = \frac{- 20 \times 5}{4} \Rightarrow u = - 25 c m T h e o b j e c t s h o u l d b e p l a c e d a t a d i s \tan c e 25 c m f r o m t h e c o n c a v e m i r r o r .

Regards,