Combination Of Springs

When two springs act in parallel on the same mass, each contributes its own restoring force for a common displacement, so the effective stiffness adds: $k_{eff}=k_1+k_2$. Here $k_{eff}=300+300=600\text{ N/m}$. The oscillation frequency is $f=\frac{1}{2\pi }\sqrt{k_{eff}/m}$, which gives $f=\frac{1}{2\pi }\sqrt{600/2}=\frac{1}{2\pi }\sqrt{300}=\frac{17.32}{6.283}\approx 2.76\text{ Hz}$. The value $1.95\text{ Hz}$ results from treating the springs as a single 300 N/m spring, ignoring the parallel combination. The value $3.90\text{ Hz}$ doubles the correct frequency by mistakenly using $4k$. The value $0.38\text{ Hz}$ inverts the result, reporting a period-like number in place of frequency. This applies the NCERT rule that parallel springs add their constants while series springs add reciprocals. As a sanity check, adding a second spring stiffens the system, so the frequency must exceed the single-spring value of about $1.95\text{ Hz}$, and $2.76\text{ Hz}$ correctly reflects the factor $\sqrt{2}$ increase.