Average weight of a group of children

The average weight of a group of children is 100 pounds. Todd, who weighs 112 pounds, then joins the group. This raises the average weight of the group to 102 pounds. How many children were there in the original group?

Let N be the number of children in the group. W be the total weight of children.

=> Average weight = W/N = 100 i.e; W = 100N

When Todd is added to the group, average weight is 102 pounds.
=>(W+112)/(N+1)=102

Substituting for W;
100N+112=102(N+1)
100N+112=102N+102
=>2N=10
=>N=5

There were 5 children in the original group.