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Question

While solving a mathematical problem, Atul squared the initial number and then subtracted $15$ from it. Pratul first subtracted $15$ from the initial number and then squared the difference. If both obtained the same answer, what was the initial number?

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

Solving for the Initial Number

Let the initial number be represented by the variable $x$.

Atul's Calculation

Atul first squared the initial number, resulting in $x^2$. Then, he subtracted $15$ from it. His result is:

$x^2 - 15$

Pratul's Calculation

Pratul first subtracted $15$ from the initial number, giving $(x - 15)$. He then squared this difference. His result is:

$(x - 15)^2$

Equating the Results

The problem states that both Atul and Pratul obtained the same answer. Therefore, we can set their results equal to each other:

$x^2 - 15 = (x - 15)^2$

Step-by-Step Equation Solution

  1. Expand the right side of the equation: $(x - 15)^2 = x^2 - 2(x)(15) + 15^2 = x^2 - 30x + 225$.
  2. Substitute the expanded form back into the equation: $x^2 - 15 = x^2 - 30x + 225$.
  3. Subtract $x^2$ from both sides: $-15 = -30x + 225$.
  4. Add $30x$ to both sides: $30x - 15 = 225$.
  5. Add $15$ to both sides: $30x = 225 + 15$.
  6. Simplify: $30x = 240$.
  7. Divide by $30$: $x = \frac{240}{30}$.
  8. Calculate the final value: $x = 8$.

The initial number is $8$.

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