pl. answer..

Given, height of the object = 15 cm

             height of the image = 20 cm

             Image distance(v) = 10 cm

We have,

                  Image distance / object distance = height of the image / height of the object

so,   object distance = Image distance  x (height of the object / height of the image )

                                     = (10 x 15) / 20

                                     = 7.5 cm

Now using lens formula, focal length can be calculated;

i.e.  1 / f = 1 / v + 1/ u