There are 100 students in class 40 of them are girls, the average marks of the boys in science is 75% and that of girls is 65% find the average marks of class in science.

total number of students = 100
number of girls = 40
number of boys = 100-40 = 60
average marks of boys in science = 75%
let the maximum marks in science be 100.
sum of the total marks secured by boys = 60*75= 4500
average marks of girls = 65%
sum of the marks secured by girls = 40*65 = 2600
thus the average marks of the class
$$\begin{gathered} =\frac{4500+2600}{100} \\ =71\% \end{gathered}$$

hope this helps you