Correct Answer - Option 1 : 15.5 years

**Given:**

Average age of 60 students = 18 years

Average age of 25 boys = 20 years

Average age of 25 girls = 17 years

**Formula used:**

Average = sum of observations/total number of observations

**Calculation:**

Total age of the class = 18 × 60 = 1080 years

Total age of 25 boys = 25 × 20 = 500 years

Total age of 25 girls = 25 × 17 = 425 years

Age of remaining 10 students = 1080 - (500 + 425) = 155

Average age = 155/10 = 15.5 years

**∴ The average age is 15.5 years**