In a class of  60 students, 25% are girls and the remaining are boys. If the average marks of the girls is 35 and that of the boys is 21, what is the average of the whole class?

24.5

  25% = \(\frac{1}{4}\)

             Boys      :        Girls

              3          :          1       =    4

              ↓ ×15               ↓ ×15       ↓ ×15 

             45                   15             60

Avg. of whole class = \(\frac{(45 × 21) + (15 × 35)}{60}\) = \(\frac{1470}{60}\) = 24.5