The number of students in a class is 45, out of which 331/3\% are boys and the rest are girls. The average score of girls is Science is 662/3\% more than that of boys. If the average score of all the students is 78, then the average score of girls is:

Answer: Option D

Number of students = 45
Number of boys = $$33\frac{1}{3}\%$$  of 45 = 15
Number of girls = 45 - 15 = 30
Obtain marks of girls in science subject is $$66\frac{2}{3}\%$$  more then the obtain marks of boys.
$$\begin{array}{rl}& 66\frac{2}{3}\%=\frac{2}{3}\\ & \frac{\text{Girls}}{\text{Boys}}=\frac{5x}{3x}\end{array}$$
Total marks
15 × 3x + 30 × 5x = 45 × 78
3x + 2 × 5x = 3 × 78
13x = 3 × 78
x = 3 × 6
x = 18
5x = 18 × 5
5x = 90