The average marks of a class of 30 students is 40 and that of another class of 50 students is 70. Find the average marks of all the students?	Choose the correct choice:	A) 52.5	B) 52.9	C) 52.1	D) 58.75	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 * 70 = 3500 sum of the marks for the class of 80 students = 1200 + 3500 = 4700 average marks of all the students = 4700 / 80 = 58.75 answer : d"
Answer: D