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Question

If 8 : 4 :: 3.2 : x, what is the value of x?

This question was previously asked in
RRB NTPC 2024 Undergraduate CBT 1 Question Paper (29-Aug-2025) (Shift 1)
The correct answer is

1.6

Solving Proportion 8 : 4 :: 3.2 : x - Find the Value of x

This question involves solving a proportion, which is a statement that two ratios are equal. The given proportion is:

\(8 : 4 :: 3.2 : x\)

This can be read as "8 is to 4 as 3.2 is to x".

Understanding Proportions

A proportion sets two ratios equal to each other. Mathematically, if we have a proportion like \(a : b :: c : d\), it can be written as a fraction:

\( \frac{a}{b} = \frac{c}{d} \)

In our case, \(a = 8\), \(b = 4\), \(c = 3.2\), and \(d = x\).

Step-by-Step Calculation

  1. Set up the equation:

    Using the values from the question, we write the proportion as an equation:

    \( \frac{8}{4} = \frac{3.2}{x} \)

  2. Simplify the known ratio:

    The ratio on the left side, \(8/4\), can be simplified:

    \( \frac{8}{4} = 2 \)

    So, the equation becomes:

    \( 2 = \frac{3.2}{x} \)

  3. Solve for x:

    To find the value of \(x\), we can rearrange the equation. Multiply both sides by \(x\):

    \( 2 \times x = 3.2 \)

    \( 2x = 3.2 \)

    Now, divide both sides by 2 to isolate \(x\):

    \( x = \frac{3.2}{2} \)

  4. Calculate the final value:

    Performing the division:

    \( x = 1.6 \)

Conclusion

By setting up the proportion and solving the equation step-by-step, we find that the value of \(x\) is 1.6.

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Important Questions from Ratio and proportion

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  3. How many different sums can be formed with the denominations Rs. 50, Rs. 100, Rs. 200, Rs. 500 and Rs. 2,000 taking at least three denominations at a time?

  4. In a 500 metres race, B starts 45 metres ahead of A, but A wins the race while B is still 35 metres behind. What is the ratio of the speeds of A to B assuming that both start at the same time?

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