The average marks of a class of 30 students is 40 and that of another class of 50 students is 90. Find the average marks of all the students?  Choose the correct answer:  A) 71.25  B) 52.9  C) 52.1  D) 52.3  E) 42.5

Rationale: "sum of the marks for the class of 30 students = 30 * 40 = 1200 sum of the marks for the class of 50 students = 50 * 90 = 4500 sum of the marks for the class of 80 students = 1200 + 4500 = 5700 average marks of all the students = 5700 / 80 = 71.25 answer : a"
Answer: A