Shift by glass slab `=3(1-1//3//2)=1cm`
The mirror is shifted by `1cm` towards slab.
Distance of object from vertical mirror `=20cm`
Image by virtual concave mirror
`u=-20cm, f=-5cm, v=?`
`1/v+1/u=1/f`
`1/v+(1)/(-20)=(1)/(-5)`
`1/v=-1/5+(1)/(20)=(-4+1)/(20)`
`v=-(20)/(3)cm`
Distance of I from original mirror
`=20/3+1=(23)/(3)cm`