An object is placed at a distance of 40 cm from a concave mirror of...

By mirror formula,${v}_{1} = \dfrac{uf}{u - f} = \dfrac{\left(- 40\right)\left(- 15\right)}{- 40 + 15} = \dfrac{600}{- 25} = - 24cm$



When object is displaced by 20 cm towards mirror ${v}_{2} = \dfrac{uf}{u - f} = \dfrac{\left(- 20\right)\left(- 15\right)}{- 20 + 15} = - 60cm$

Displacement of image $= {v}_{2} - {v}_{1} = - 36cm$

= 36 cm away from the mirror