Explanation:

- There are 50 students in total: 40% are girls. This implies there are 20 girls (since 40% of 50 = 20) and 30 boys.

- The average score for all students is 66.

- Let the average score of the girls be \( x \). The average score of the boys is then 20% less, which is \( 0.8x \).

- We use the equation: \((20 \times x) + (30 \times 0.8x) = 50 \times 66\).

- Simplifying gives: \(20x + 24x = 3300\), leading to \(44x = 3300\).

- Solving for \( x \) gives \( x = 75 \).

- Option 1: 80: Too high compared to calculated 75.

- Option 2: 65: Lower than needed and doesn't solve equation.

- Option 3: 75: Solves the equation accurately.

- Option 4: 70: Lesser than required to reach average of 66.