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Both the numerator and the denominator of \(\frac{3}{4}\) are increased by a positive integer, \(x\), and those of \(\frac{15}{17}\) are decreased by the same integer. This operation results in the same value for both the fractions.

What is the value of \(x\)?

  • A.1
  • B.2
  • C.3
  • D.4

Answer: C

\(x = 3\).

After the change the two fractions are \(\frac{3 + x}{4 + x}\) and \(\frac{15 - x}{17 - x}\). Setting them equal and cross-multiplying:

\[ \begin{aligned} (3 + x)(17 - x) &= (15 - x)(4 + x) \\ 51 + 14x - x^{2} &= 60 + 11x - x^{2} \\ 3x &= 9,\quad x = 3 \end{aligned} \]

Check: \(6/7\) and \(12/14 = 6/7\). With 1, 2 or 4 the fractions differ, for example \(4/5\) against \(14/16 = 7/8\) for x = 1.

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