The variation of potential energy of a harmonic oscillator is as...

According to figure, when (y=20 \mathrm{~mm}) (=2 \times 10^{-2} \mathrm{~m}) then (U_{\max }=0.04 \mathrm{~J}=4 \times 10^{-2} \mathrm{~J})
When $y=0$ then (U_{\min }=0.01 \mathrm{~J}=1 \times 10^{-2} \mathrm{~J}) (\therefore) The change in potential energy, ($$\begin{array}{ll} & U_{\max }-U_{\min }=\frac{1}{2} K y^2 \\ \Rightarrow & 4 \times 10^{-2}-1 \times 10^{-2}=\frac{1}{2} K \times\left(2 \times 10^{-2}\right)^2 \\ \Rightarrow & 3 \times 10^{-2}=K \times 2 \times 10^{-4} \\ \Rightarrow & \quad K=\frac{3 \times 10^{-2}}{2 \times 10^{-4}}=1.5 \times 10^2=150 \mathrm{Nm}^{-1} \end{array}$$)