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.**