Correct Answer - R = 75 cm
Image due to plane mirror will form at a distance of `20 cm` left of the mirror.
Since image formed by two mirrors lie adjacent to each other.
For convex mirror, image position is `15 cm` towards left
`u=-25 cm rArr v=+15 cm`
using `(1)/(v)+(1)/(u)=(1)/(f)=(2)/(R)`
`(1)/(15)-(1)/(25)=(2)/(R) R =75 cm`.