A number $n$ is added to $3, 7, 9, 12$ so that the mean of the new $5$ numbers is $9$. Find $n$.

A number $n$ is added to $3, 7, 9, 12$ so that the mean of the new $5$ numbers is $9$. Find $n$.
  1. $10$
  2. $12$
  3. $14$
  4. $15$

Solution

$\dfrac{3+7+9+12+n}{5} = 9 \Rightarrow 31+n = 45 \Rightarrow n = 14$.

Asked in: IMO

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