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Question

A container has 60 litres of a mixture of acid and water in the ratio 7:3. 20% of the mixture is taken out and replaced with water. This operation is repeated once more. What is the final percentage of acid in the mixture?

This question was previously asked in
SSC CGL 2025 Tier 2 Paper 1 Question Paper (19-Jan-2026)
The correct answer is

44.8%

Final Acid Percentage Calculation

The question asks for the final percentage of acid in a 60-litre mixture after a specific operation is performed twice. The operation involves removing 20% of the mixture and replacing it with water.

Initial Acid Concentration

First, determine the initial amount and concentration of acid.

  • Total initial mixture: 60 L
  • Initial ratio of Acid to Water: 7:3
  • Total ratio parts: $7 + 3 = 10$

Calculate the initial quantity of acid:

Initial Acid Quantity = $\frac{\text{Acid parts}}{\text{Total parts}} \times \text{Total Volume}$

Initial Acid Quantity = $\frac{7}{10} \times 60 \text{ L} = 42 \text{ L}$

Calculate the initial concentration of acid:

Initial Acid Concentration = $\frac{\text{Initial Acid Quantity}}{\text{Total Volume}} \times 100\%$

Initial Acid Concentration = $\frac{42 \text{ L}}{60 \text{ L}} \times 100\% = 0.70 \times 100\% = 70\%$

Acid Concentration After Replacements

Each operation involves removing 20% of the current mixture and adding water. This means 80% of the existing mixture remains.

The fraction of the acid remaining after one such operation is $(1 - 0.20) = 0.80$.

This operation is repeated once more, meaning it occurs a total of two times (N=2).

The formula to find the final concentration of the component (acid) is:

$ \text{Final Concentration} = \text{Initial Concentration} \times (1 - \text{Fraction Removed})^N $

Calculation

Using the formula with Initial Concentration = 0.70, Fraction Removed = 0.20, and N = 2:

$ \text{Final Acid Concentration} = 0.70 \times (1 - 0.20)^2 $

$ \text{Final Acid Concentration} = 0.70 \times (0.80)^2 $

$ \text{Final Acid Concentration} = 0.70 \times 0.64 $

$ \text{Final Acid Concentration} = 0.448 $

Convert the final concentration to a percentage:

$ \text{Final Acid Percentage} = 0.448 \times 100\% = 44.8\% $

Therefore, the final percentage of acid in the mixture is 44.8%.

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Important Questions from Mixture and Alligation

  1. A bottle contains 20 litres of liquid A. 4 litres of liquid A is taken out of it d replaced by same quantity of liquid B. Again 4 litre 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?

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  3. If 1 litre of water weighs 1 kg, then how many cubic millimeters of water will weigh 0.1 gm?

  4. A vessel full of water weighs 4o kg. If it is one-third filled, its weight becomes 20 kg. What is the weight of the empty vessel?

  5. Two equal glasses of same type are respectively 1/3 and 1/4 full of milk. They are then filled up with water and the contents are mixed in a pot. What is the ratio of milk and water in the pot?

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