The average weight of a group of children is 100 pounds. todd, who weighs 102 pounds, then joins the group. this raises the average weight of the group to 102 pounds. how many children were in the original group?

First create your let statements to represent the unknown.
1. We don't know how many children are in the group, so let us make that amount be equal to x.
2. We also don't know the weight of the individual children, let us make that number be equal to y.
Now in order to find the average of anything, you add the total and then divide it by the number of things we are averaging.
We know the average so we can make this statement.
x/y = 100
...
We must isolate y to find the weight of the individual children, this gives us this:
y=100x
Now we must add Todd to our new equation
y+102/x+1 = 102
Let me explain the above, the reason why we add 102 to the unknown weight of the children is because that is the added weight.
The reason why we add 1 to x is because 1, adds to the unknown number of children.
We will sub in the weight of the average children as 100, this will replace y.
You get this so far:
100x+102/x+1 = 102
Now we solve to get x
The end answer is 0/202 = x when you isolate so, I believe you have a typo.