Explanation: Solve: \(\lambda = \frac{0.693}{T_{1/2}} = \frac{0.693}{15} = 0.0462\) \(N = N_0 e^{-\lambda t}\) \( \implies 40\% \, \text{of} \, N_0 = N_0 e^{-\lambda t}\) \( \implies 0.4 N_0 = N_0 e^{-\lambda t}\) \( \implies \ln (0.4) = -\lambda t \, \text{(উভয়পক্ষে লন নিয়ে)}\) \( \implies t = \frac{\ln (0.4)}{-0.0462} = 19.83 \, \text{দিন}\) Ans. (C)