Correct Answer: 20 yr
Explanation:
Total age of 5 sister = 20 x 5 = 100 yr.
4 yr ago, total sum of ages
= 100 - ( 5 x 4) = 100 - 20 = 80 yr
But at that time (4 yr ago), there were 4 sister in the group.
∴ Average age at that time(4 yr ago) = 80/4 = 20 yr.
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