Correct Answer - Option 3 : 10
Given:
The average age of 30 girls = 13
The average age of the first 13 girls = 15
Calculations:
Total age of 30 girls = 30 × 13
⇒ 390
Total age of 18 girls = 15 × 18
⇒ 270
∴ age of the remaining 12 girls = 390 – 270
⇒ 120
∴ required average = 120/12
⇒ 10 years
The average age of the remaining 12 girls is 10 years.