Question

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

Answer 1

$Two$ years ago the mean of $40$ people was $11$ years.

So, total age $two$ years ago $=40\times 11=440$ $years.$

Now, after $two$ years the total age that would increase is $=40\times 2=80$ $years.$

$\therefore$ Total age of present $=440+80=520$ $years.$

Now, when $one$ person left, mean age is $12$ $years.$

$\therefore$ Total age after $1$ person left $=39\times 12=468$ $years.$

$\therefore$ The person who left is $520-468=52$ $years$ old.