The average marks of a class of 20 students is 40 and that of another class of 50 students is 60. Find the average marks of all the students?	Choose the correct choice from the following choices.
A) 44.28
B) 54.28
C) 34.28
D) 64.28
E) 74.28

Rationale: "sum of the marks for the class of 20 students = 20 * 40 = 800 sum of the marks for the class of 50 students = 50 * 60 = 3000 sum of the marks for the class of 70 students = 800 + 3000 = 3800 average marks of all the students = 3800 / 70 = 54.28 answer : b"
Answer: B