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Add a pure component to a mixture to reach a target ratio or percent

This type of question, asked 3 times: CDS 1 2018, CDS 1 2021, CDS 1 2025. All 3 questions below, oldest first.

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  1. Question 1

    CDS 1 2018

    42
    An alloy A contains two elements, copper and tin in the ratio of 2:32 : 3, whereas an alloy B contains the same elements in the ratio of 3:43 : 4. If 20 kg of alloy A, 28 kg of alloy B and some more pure copper are mixed to form a third alloy C which now contains copper and tin in the ratio of 6:76 : 7, then what is the quantity of pure copper mixed in the alloy C?

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  2. Question 2

    CDS 1 2021

    38
    In a mixture of 80 litres of a liquid and water, 25% of the mixture is the liquid. How much water should be added to the mixture so that the liquid becomes 20% of the mixture?

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  3. Question 3

    CDS 1 2025

    10
    A mixture of 100 L contains kerosene and turpentine oil in the ratio 3 : 2. What is the minimum quantity of kerosene in litres (whole number) that should be mixed in the mixture so that the resulting mixture has 20% of kerosene ?

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