Explain that to add using partial sums, you put the numbers one under another. Then you can add them together by solving from left to right. State that you put the largest number of the sum on the top and the other number underneath it. For partial sums, you work from left to right. So you start with adding the hundreds numbers together. Then you calculate how much the tens numbers are together and then you add the ones values together. In adding the ones numbers up, it can happen that you go into the tens values. In that case you write the number in the tens number space and the ones value space. You put all these numbers in the diagram and you state what the intermediate sums were. If you add the numbers from the diagram together, then you find the answer. You put this answer at the bottom. Next there are two sums in diagrams, in which different rows are in yellow. Ask the students if they know which intermediate sum belongs in each colored row. This is how you can check whether students know which steps they have to take. Then you have the students solve a few problems with partial sums. Next you explain that you can also add above the hundreds numbers. In that case you write the number in the spaces for the hundreds numbers, the tens numbers and the ones values. Then practice partial sums in two problems for which for one, part of the diagram is already filled in and for the other, the diagram still must be filled in.

Check whether the students can add using partial sums using the following questions/exercise:

- Why is it useful to be able to add using partial sums?

- What do the letters H T O mean?

- In solving partial sums, where do you always start?

- Calculate these using partial sums: 436 + 66 and 287 + 162 + 131