The average age of a group of 6 sisters decreases by 12 years immediately when a pair of twins is born. What is the total age of all 8 siblings immediately after the birth of the twins?
Solution
Let the average of all the sisters before the birth of the pair of twins is X.
So, the sum of their ages = 6X
When the pair of twins born, the sum of their ages would remain the same.
Hence, the average of their ages when the pair of twins born = 6X/8 or 3X/4
So, the average decreased by 12 i.e. X - 3X/4 = 12
X/4 = 12
Or, X = 48
Therefore, the sum of their ages when the pair of twins born = 6*48 = 288
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