The average age of 6 sisters is 30 yr. If the age of the youngest sister be 5 yr. What was the average age of the group of sister just before the birth of the youngest sister?

Total age of 6 sisters = 30 × 6 = 180 years 5 years ago, Total sum of ages = 180 – (5 × 6) = 150 years But at that time (5years ago), there were 5 sisters in the group ∴ Average age at that time (5 years ago) = 150/5 = 30 years Hence, the required answer is 30 years.

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