The average marks of a class of 30 students is 40 and that of another class of 50 students is 80. Find the average marks of all the students? Choose the correct choice from the following choices.
A) 52.5  B) 52.9  C) 52.1  D) 52.3  E) 65

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 * 80 = 4000 sum of the marks for the class of 80 students = 1200 + 4000 = 5200 average marks of all the students = 5200 / 80 = 65 answer : e"
Answer: E