300 children are placed into 2 groups. 40 children move from the first group to the second group. 78 children move from the second group to the first group. There are now the same number of children in each group.
Which group had more children in at the start? How many more?
If the groups had x and y kids, then
x+y = 300
x-40+78 = y+40-78
x = 112, y = 188