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Question

A milkman mixes 10% water in pure milk but he is not content with it. So, he again mixes 10% more water in the previous mixture. What is the profit percentage of milk if he sells it at cost price?

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
21%

Milkman Profit Calculation with Water Mixture

This solution explains how to calculate the profit percentage when a milkman mixes water into pure milk twice and sells the mixture at the cost price.

Problem Analysis

  • A milkman adds 10% water initially.
  • He then adds another 10% water based on the previous mixture's volume.
  • The final mixture is sold at the cost price of the original pure milk.

Step-by-Step Solution

Step 1: Assume Initial Quantity and Cost

Let's assume the initial quantity of pure milk is 100 units.

Let the cost price of 1 unit of pure milk be $C.

Therefore, the initial Cost Price (CP) of the milk is:
$ \text{CP} = 100 \text{ units} \times C/\text{unit} = 100C $

Step 2: Calculate Mixture Volume after First Water Addition

The milkman first mixes 10% water in pure milk.

Water added = $ 10\% \times 100 \text{ units} = 10 \text{ units} $.

The new volume of the mixture after the first addition is:
$ \text{Volume}_1 = 100 \text{ units} (\text{milk}) + 10 \text{ units} (\text{water}) = 110 \text{ units} $

Step 3: Calculate Final Mixture Volume after Second Water Addition

He then adds 10% more water based on the previous mixture's volume (110 units).

Additional water added = $ 10\% \times 110 \text{ units} = 11 \text{ units} $.

The final volume of the mixture is:
$ \text{Final Volume} = \text{Volume}_1 + \text{Additional Water} = 110 \text{ units} + 11 \text{ units} = 121 \text{ units} $

Step 4: Determine Selling Price (SP)

The milkman sells the final mixture (121 units) at the cost price. This implies the selling price per unit volume is the same as the cost price per unit volume of pure milk ($C$).

Total Selling Price (SP) = $ \text{Final Volume} \times C/\text{unit} $
$ \text{SP} = 121 \text{ units} \times C/\text{unit} = 121C $

Step 5: Calculate Profit

Profit is the difference between the Selling Price and the Cost Price.

$ \text{Profit} = \text{SP} - \text{CP} $
$ \text{Profit} = 121C - 100C = 21C $

Step 6: Calculate Profit Percentage

The profit percentage is calculated based on the initial Cost Price.

$ \text{Profit Percentage} = \left( \frac{\text{Profit}}{\text{CP}} \right) \times 100 $
$ \text{Profit Percentage} = \left( \frac{21C}{100C} \right) \times 100 $
$ \text{Profit Percentage} = \frac{21}{100} \times 100 = 21\% $

Conclusion

By adding water twice, the milkman increases the volume. When this larger volume is sold at the original cost price per unit, it results in a significant profit.

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Similar Questions

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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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