For retracing of ray, ray must fall normally on mirror i. e., toward the center of curvature.
`(1)/(v)-(1)/(u)=(1)/(f),u=-15`
`(1)/(v)+(1)/(15)=(1)/(10),f=10cm`
`v=30 cm`
For mirror `(1)/(v) +(1)/(v)=(1)/(f),u=2f`
`(1)/(v)+(1)/(2f)=(1)/(f)rArrV=2f`
Hence, from the ray diagram,
`2f=30-10`
`f=10cm`
![image](https://learnqa.s3.ap-south-1.amazonaws.com/images/161040626111789518721610406261.png)