A convex lens of focal length 15 cm is placed coaxially on an optical bench in front of a convex mirror at a distance of 5 cm from it. It is found that an object and its image coincide, if the object is placed at a distance of 20 cm from the lens. The focal length of the convex mirror is :
Option 1) 27. 5 cm
Option 2) 20. 0 cm
Option 3) 25. 0 cm
Option 4) 30. 5 cm