Same number is added to numerator and denominator of $\dfrac{2}{5}$, giving $\dfrac{1}{2}$. The number is

Same number is added to numerator and denominator of $\dfrac{2}{5}$, giving $\dfrac{1}{2}$. The number is
  1. $1$
  2. $3$
  3. $5$
  4. $2$

Solution

$\dfrac{2+x}{5+x} = \dfrac{1}{2} \Rightarrow 2(2+x) = 5+x \Rightarrow x = 1$.

Asked in: IMO

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