There are two groups A and B of a class, consisting of 42 and 28 students respectively. If the average weight of group A is 25 kg and that of group B is 40 kg, find the average weight of whole class?
[A]30 Kg
[B]31 Kg
[C]69 Kg
[D]70 Kg

31 Kg
Average weight of both groups =
= \frac{42\times25 + 28\times 40}{42+28}
= \frac{1050 + 1120}{70} = \frac{2170}{70} = 31 kg.
Hence option [B] is correct answer.