(c) Reduced mass of the system, $\mu = \frac{m_{1}m_{2}}{m_{1} + m_{2}} = \frac{m \times 3m}{m + 3m} = \frac{3m}{4}$ Period of oscillation, $T = 2\pi\sqrt{\frac{\mu}{k}} = 2\pi\sqrt{\frac{3m/4}{k}} = \pi\sqrt{\frac{3m}{k}}$
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