Ages of two friends differ by $3$, sum $= 27$. Age of younger is

Ages of two friends differ by $3$, sum $= 27$. Age of younger is
  1. $12$
  2. $15$
  3. $10$
  4. $13$

Solution

$2x + 3 = 27 \Rightarrow x = 12$.

Asked in: IMO

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