In a class of  75 students, 40% are girls and the remaining are boys. If the average marks of the girls is 30 and that of the boys is 22, what is the average of the whole class?

25.2

  40% = \(\frac{2}{5}\)

 so ratio of               Boys      :        Girls

                                3          :          2      

( 3 + 2 )R = 75  

R = 15

Number of boys = 3 ×15 = 45 

Number of girls = 2 ×15 = 30

Avg. of whole class = \(\frac{(45 × 22) + (30 × 30)}{75}\) = \(\frac{1890}{75}\) = 25.2