The average marks of 50 students in a class is 72 and the average marks of boys and girls are 70 and 75 respectively. The number of boys in the class is

Given average marks of 50 students in a class is 72. Then total marks is given by,

T = 50 x 72 = 3600

If total number of boys are B then total girls will be (50-B).

And also average marks of boys is 70 and that of girls is 75. Then equation can be written as,

T = 70(B) + 75(50 - B) $$\Rightarrow$$ 3600 = 70B - 75B + 3750 

$$\Rightarrow$$ 5B = 150 $$\Rightarrow$$ B = 30

Hence, option D is the correct answer.