A bottle contains 20 litres of liquid A. 4 litres of liquid A is taken out of it and replaced by same quantity of liquid B. Again 4 litres of the mixture is taken out and replaced by same quantity of liquid B. What is the ratio of quantity of liquid A to that of liquid B in the final mixture?

1. **Identify the Fraction**: The bottle has 20 Litres. We remove 4 Litres. \n - Fraction removed = $\\frac{4}{20} = \\frac{1}{5}$.\n\n2. **Switch to Remaining Portion**: As learned from the **2017 Population Migration PYQ**, it is elegant to focus on what is left. \n - If $\\frac{1}{5}$ goes, $\\frac{4}{5}$ remains [5] [36].\n\n3. **Apply Successive Change**: The operation happens twice. \n - Just like the **2014 Net Effect** or **2017 Migration** questions, we multiply the factor: \n - Fraction of Liquid A left = $\\frac{4}{5} \\times \\frac{4}{5} = \\frac{16}{25}$ [32] [34].\n\n4. **Interpret the Ratio**: \n - The result $\\frac{16}{25}$ tells us that out of 25 total parts, 16 parts are Liquid A.\n - Therefore, the remaining parts must be Liquid B: $25 - 16 = 9$ parts.\n\n5. **Final Check**: The ratio of A to B is **16 : 9**. This matches Option C.