.98 m
9.8 cm
9.8 m
.009 km
The loss in gravitational potential energy would be equal to the gain in elastic potential energy. At the point of maximum deformation, all the gravitational potential energy would convert to elastic potential energy.
⟹mgh = 1/2kx²
⟹x = √(2mgh/k) = √(2 × 0.04 × 10 × 0.05 / 400) m ≈ 0.098 m = 9.8 cm