The average marks of a class of 30 students is 30 and that of another class of 50 students is 60. Find the average marks of all the students?	Pick one:  A) 52.5  B) 52.9  C) 52.1  D) 48.75  E) 42.5

Rationale: "sum of the marks for the class of 30 students = 30 * 30 = 900 sum of the marks for the class of 50 students = 50 * 60 = 3000 sum of the marks for the class of 80 students = 900 + 3000 = 3900 average marks of all the students = 3900 / 80 = 48.75 answer : d"
Answer: D