Explanation:

- There are 100 students in the class.

- 70% of the students are girls, which means there are 70 girls.

- The remaining 30 students are boys.

- The girls' average score is 20% more than the boys' average.

- The overall average score of all students is 57.

- Let's assume the average score of boys is \( x \).

- Therefore, the average score of girls would be \( 1.2x \).

- Using the formula for the total score: \( 70 \times 1.2x + 30 \times x = 5700 \).

- Simplifying gives: \( 84x + 30x = 5700 \).

- Further simplification: \( 114x = 5700 \).

- Solving for \( x \): \( x = 5700 / 114 = 50 \).

Option 3: 50 is the correct answer.