The average weight of a group of boys is 30 kg. After a boy of weight 39 kg joins the group, the average weight of the group goes up by 1 kg. Find the number of boys in the group originally ? Select:
A) 4
B) 8
C) 5
D) 2
E) 9
Let the number off boys in the group originally be x.   Total weight of the boys = 30x   After the boy weighing 39 kg joins the group, total weight of boys = 30x + 39   So 30x + 39 + 31(x + 1) = &gt; x = 8.   Answer: 8
<result>B</result>
The average weight of a group of boys is 30 kg. After a boy of weight 39 kg joins the group, the average weight of the group goes up by 1 kg. Find the number of boys in the group originally ?