\(K _{ i } =0\)
\(U _{ f }=\frac{1}{2} k \left(\frac{ x _0}{2}\right)^2\)
\(K _{ f }=0.25\,J\)
\(\frac{1}{2} kx _0^2+0=\frac{1}{2} k \frac{ x _0^2}{4}+0.25\)
\(\frac{1}{2} k x _0^2 \frac{3}{4}=\frac{1}{4}\)
\(\frac{1}{2} k \frac{3}{100}=1 \Rightarrow k =\frac{200}{3}\,N / m\)
\(=67\,N / m\)