Ten years ago, father was $5$ times as old as son. Today sum of ages is $50$. Son's age today is

Ten years ago, father was $5$ times as old as son. Today sum of ages is $50$. Son's age today is
  1. $15$
  2. $10$
  3. $20$
  4. $5$

Solution

$50 - s - 10 = 5(s - 10) \Rightarrow 40 - s = 5s - 50 \Rightarrow 90 = 6s \Rightarrow s = 15$.

Asked in: IMO

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