Here, the real image is formed on the wall which is at a distance of 12 cm from the convex lens. This means that the distance of image from the convex lens or image distance will be 12 cm. Since a real image is fomred on the right side of the lens, so this image distance will be positive.
Now, Image distance, `v=+- 12` cm (A real image)
Object distance, u=? (To be calculated)
Focal length, f=+10 cm (it is convex lens)
Putting these values in the lens formula:
`1/v-1/u=1/f`
We get: `1/12-1/u=1/10`
`1/12-1/10=1/u`
`(5-6)/(60)=1/u`
`-1/60=1/u`
So, Object distance, u=-60 cm
Thus, the obect should be place at a distance of 60 cm in front of the convex lens. The minus sign shows that the obect is on the left side of the lens.