Explanation: Solve: \frac{1}{f} = (\mu - 1) \left( \frac{1}{r_1} - \frac{1}{r_2} \right)
\implies \frac{1}{0.25} = (1.6 - 1) \left( \frac{1}{0.2} + \frac{1}{r_2} \right)
\implies \left( \frac{1}{0.2} + \frac{1}{r_2} \right) = \frac{1}{0.25 \times 0.6}
\implies -\frac{1}{r_2} = \left( \frac{1}{0.2} - \frac{1}{0.25 \times 0.6} \right)
\implies r_2 = + 0.6 \, \text{m} \therefore r_2 = 60 \, \text{cm} Ans. (D)
