Class composition problem: Deduce total students and compute number of girls. Context: 18 boys over 160 cm are three-fourths of all boys. Boys constitute two-thirds of the total students in the class. Find: Number of girls in the class. Choose the correct value.

Given data

• 18 boys = (3/4) of total boys ⇒ Total boys = 18 ÷ (3/4) = 18 × (4/3) = 24
• Boys = (2/3) of total students ⇒ Total students = 24 ÷ (2/3) = 24 × (3/2) = 36

Compute girls Girls = Total students − Boys = 36 − 24 = 12

Common pitfalls Dividing by 3/4 as 24 × 3/4 instead of 24 ÷ 3/4 = 24 × 4/3.