Question

Two years ago the mean age of $40$ people was $11$ years. Now a person left the group and the mean age is is changed to $12$ years. Find the age of the person who left the group.

Answer 1

$\Rightarrow$ Two years ago the mean age for $40$ people was $11$ years.

$\therefore$ Total age of $40$ people two years ago $=40\times 11=440$

Now, after two years the total age should go up $=(2\times 40)+440=80+440=520.$

$\Rightarrow$ When one person left, the for $39$ people the mean age is $12.$

$\Rightarrow$ Total age of $39$ people $=39\times 12=468$

$\Rightarrow$ Required age $=520-468=52years$