Father is $25$ years older than son. After $5$ years, father will be twice son's age. Son's present age is

Father is $25$ years older than son. After $5$ years, father will be twice son's age. Son's present age is
  1. $20$
  2. $25$
  3. $15$
  4. $30$

Solution

$s + 30 = 2(s + 5) \Rightarrow s + 30 = 2s + 10 \Rightarrow s = 20$.

Asked in: IMO

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