New Average when a person leaves the group

Rule 1:

In case of increase in average
Age of leaving member = Previous average - ( Increase in average × Number of members excluding leaving member)

Example

The average weight of 10 mangos in the box is 450 grams. But accidentally one mango fell away out of the box resulting in increase of the average weight by 10 grams. What is the weight of that mango?

Sol:

Age of leaving member = Previous average - ( Increase in average × Number of members excluding leaving member)

Previous average = 450 grams

Increase in average = 10 grams

Number of members excluding leaving member = 9

Age of leaving member = 450 – (10 × 9) = 450 – 90 = 360 grams.

Rule 2:

In case of decrease in average
Age of leaving member = Previous average + ( Decrease in average × Number of members excluding leaving member)

Example

The average height of 10 boys is 165 cm. But one boy leaves the class and as a result there is a decrease in average height by 2 cm. What is height of leaving boy?

Sol:

Age of leaving member = Previous average + ( Decrease in average × Number of members excluding leaving member)

Previous average = 165 cm

Decrease in average = 2 cm

Number of members excluding leaving member = 9

Age of leaving member = 165 + ( 2 × 9) = 165 + 18 = 183 cm

Rule 3:

New average of group =

Example

A group of 10 persons has average age of 30 years. A person aged 40 years left the group. What is new average of the group?

Sol:

New average of group =

Previous average = 30 years

Number of persons including leaving person = 10

Age of leaving member = 40 years

Number of members excluding leaving member = 9

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