You can't imagine my surprise when I learned that there was an APP for
Partial Sums Addition
on my iPhone! Partial Sums is taught to third graders through the Everyday Mathematics to make adding 2-digit and 3-digit addition easier. To me and other adults who learned addition in the traditional method, it's definitely confusing! But once you get the hang of it you realize why they are being taught this way.
So if you are helping your child with their Math homework and you are very confused, you should consider buying this app for your iPhone! It's definitely worth $3.99!
Partial Sums tutorial and practice worksheets... http://www.unit5.org/mrozj/math/partsums.htm