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Question

A sample of milk from a vessel contains 4% water. What quantity of pure milk should be added to 8 L of milk in the vessel to reduce the water content to 2%?

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
8 L

Calculating Pure Milk Addition for Water Reduction

This problem involves calculating the amount of pure milk to add to a mixture to achieve a specific lower water concentration. We need to find the quantity of pure milk to add to 8 L of milk, initially containing 4% water, so that the final mixture has only 2% water.

Initial Mixture Analysis

  • Total initial volume of milk = 8 L
  • Initial water percentage = 4%
  • Calculate the initial amount of water:
    Amount of Water = $4\% \times 8 \text{ L} = 0.04 \times 8 \text{ L} = 0.32 \text{ L}$
  • The amount of pure milk initially is $8 \text{ L} - 0.32 \text{ L} = 7.68 \text{ L}$.

Calculating Added Pure Milk

Let the quantity of pure milk added be '$x$' litres.

  • When pure milk is added, the amount of water remains constant ($0.32$ L), but the total volume increases.
  • New total volume = $(8 + x)$ L
  • The desired final water percentage is 2%.
  • We can set up an equation using the final state:
    $\frac{\text{Amount of Water}}{\text{New Total Volume}} = \text{Desired Water Percentage}$
    $\frac{0.32 \text{ L}}{(8 + x) \text{ L}} = 2\% = 0.02$

Solving for Added Milk Quantity

  1. Cross-multiply to solve the equation:
    $0.32 = 0.02 \times (8 + x)$
  2. Distribute the 0.02:
    $0.32 = (0.02 \times 8) + (0.02 \times x)$
    $0.32 = 0.16 + 0.02x$
  3. Isolate the term with '$x$':
    $0.02x = 0.32 - 0.16$
    $0.02x = 0.16$
  4. Solve for '$x$':
    $x = \frac{0.16}{0.02} = \frac{16}{2} = 8$

Therefore, 8 L of pure milk should be added.

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Important Questions from Mixture and Alligation

  1. My father is presently 25 years older than me. The sum of our ages 5 years ago was 39 years. Find my present age.

  2. Rice worth ₹43/kg and ₹67/kg are mixed with a third variety in the ratio 2 : 1 : 5. If the mixture is worth ₹96/kg, the price (in ₹) of the third variety of rice per kg will be:

  3. 42 litres of milk at ₹25 per litre is mixed with 28 litres of milk at ₹40 per litre. Find the average price of the mixture.

  4. 15 kg of ₹10 per kg wheat is mixed with 5 kg of another type of wheat to get a mixture costing ₹30 per kg. Find the price (per kg) of the costlier wheat.

  5. If a + b + c = 0, then the value of (a² + b² + 2ab) is equal to:

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