Question

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

Answer 1

The Mean age of 40 people = 11 years
The Mean age of 40 people
= A1 + A2 + A3 ...... + A40
____________________
40
$\frac{a1 + a2....a40}{40} = 11$
$a1 + a2....a40 = 11 \times 40$
$a1 + a2....a40 = 440$
The mean age of 39 people = 12 years
$\frac{a1 + a2.....a39}{39} = 12$
$a1 + a2.....a39 = 12 \times 39$
$a1 + a2.....a39 = 468$
A1+A2 ...... A39+ A40 = 440
468 + A40 = 440
A40 = 468 - 440
A40 = 18 years