There are 2 sections A and B in a class, consisting of 50 and 70 students respectively. If the average weight of section A is 50kg and that of section B is 70kg, Find the average of the whole class?  Choose the correct answer.
A) 50.78 kg  B) 49.32 kg  C) 61.67 kg  D) 69.15 kg  E) 70.89 kg

Rationale: "total weight of 50 + 70 students = 50 * 50 + 70 * 70 = 7400 average weight of the class is = 7400 / 120 = 61.67 kg answer is c"
Answer: C