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Question

A sample of milk contains 5% of water. What quantity of pure milk should be added to 10 L of milk sample to reduce this to 2%?

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

This problem involves calculating the amount of pure milk to add to a mixture to decrease the water percentage.

Initial Mixture Analysis

The initial milk sample has a total volume of 10 L.

  • Water percentage = 5%
  • Amount of water = $5\%$ of 10 L $= \frac{5}{100} \times 10 = 0.5$ L
  • Amount of pure milk = Total Volume - Water Amount $= 10 - 0.5 = 9.5$ L

Adding Pure Milk

Let $x$ represent the quantity (in Liters) of pure milk added to the sample.

  • The amount of water remains constant at 0.5 L.
  • The amount of pure milk increases to $9.5 + x$ L.
  • The new total volume becomes $10 + x$ L.
  • The target water percentage is 2%.

Calculating Added Milk Quantity

The concentration of water in the new mixture is expressed as:

$ \text{Water Percentage} = \frac{\text{Amount of Water}}{\text{New Total Volume}} \times 100 $

We set this to the desired 2%:

$ \frac{0.5}{10 + x} = \frac{2}{100} $

Now, we solve for $x$:

  1. Simplify the fraction: $\frac{0.5}{10 + x} = 0.02$
  2. Cross-multiply: $0.5 = 0.02 \times (10 + x)$
  3. Distribute: $0.5 = 0.2 + 0.02x$
  4. Isolate the term with $x$: $0.5 - 0.2 = 0.02x \implies 0.3 = 0.02x$
  5. Solve for $x$: $x = \frac{0.3}{0.02} = \frac{30}{2} = 15$ L

Therefore, 15 L of pure milk must 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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