What is the focal length of the mirror when a person stands 40cm in front of a cosmetic mirror (concave mirror?

15) When a person stands 40 cm in front of a cosmetic mirror (concave mirror), the erect image is twice the size of the object. What is the focal length of the mirror? how do I solve?

A) 27 cm
B) 40 cm
C) 80 cm
D) 160 cm