Question

small object space placed in front of convex lens of 5 cm focal length add a virtual image is formed at distance of 25cm the magnification

Answer 1

I/f=1/v-1/u
1/5=1/-25-1/u
-1/25-1/5=1/u
(-1-5)/25=1/u
-6/25=1/u. U=-25/6
Magnification=v/u
-25/(-25/6)=6

Answer 2

Given: Convex Lens
            v= -25 cm (Virtual)
            f = 5 cm

To find: Magnification

Proof:Using Lens Formula 
             1/u = 1/v -1/f
              1/u = 1/-25-1/5
              1/u = (1-5)/25
              1/u= -4/25
               u = -25/4

On Applying Magnifiction Formula
         m = v/u
          m = (-25/4)/-25  
        m = (25/4)/25
         m = 25/25*4
         m = 25/100
             m = 0.25 ...............................Ans