Option 2 : 128.33 cm

**Given:**

Total number of students = 60

Number of girls (G) = Number of boys (B) + 30

**Formula used**

When two groups are given with the number of items and with its average, then the combined average is calculated as follows:

Average = (Number of items in first group × average of first group + Number of items in second group × average of second group)/(number of items in first group + number of items in second group)

**Calculation**

Let the number of boys be B

⇒ The number of girls is (B + 30)

Average height of class = 130 cm

Average height of boys = 135 cm

G + B = 60

⇒ B + 30 + B = 60

⇒ 2B = 30

B = 15

⇒ 130 = [B × 135 + (B + 30) × h]/60

⇒ 7800 = 135B + Bh + 30h

⇒ 7800 = 135 × 15 + 15 × h + 30h

⇒ 7800 - 2025 = 45h

⇒ 5775 = 45h

⇒ 128.33 = h

**∴**** Average height of girls is 128.33 cm**