Father is $4$ times as old as son. After $5$ years, father will be $3$ times as old. Present age of son is

Father is $4$ times as old as son. After $5$ years, father will be $3$ times as old. Present age of son is
  1. $10$
  2. $8$
  3. $12$
  4. $15$

Solution

Son $= x$, father $= 4x$. After 5 yrs: $4x + 5 = 3(x + 5) \Rightarrow x = 10$.

Asked in: IMO

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