There are 100 students in a class, out of which 70% are girls and others are boys. The average score of girls in a test is 20% more than that of boys. If the average score of all the students is 57, then what is the average score of girls?

Given, 

Total number of Students in a class = 100 (given)

Number of girls in class = 70 (70% of total)

Number  of boys in class = 30

Average of class = 57 

Let the average score of boys = x.

Average score of girls = 1.2x (20% more than average score of boys)

As we know, 

$$Average=\frac{Sum\ of\ observation}{Number\ of\ observation}$$ 

According to question, 

$$\therefore\ \frac{\left(1.2x\times\ 70+30\times\ x\right)}{100}=57$$

$$\therefore\ \left(114x\right)=5700$$

$$\therefore\ x=50$$

So, Average Score of girls = 1.2x = 1.2$$\times\ $$50 = 60

Hence, option A is correct.