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) ⇒ 3600 = 70B - 75B + 3750 ⇒ 5B = 150 B = 30 Hence, option D is the correct answer.