Some stories have no change. They have two parts and a whole, like red and blue balloons in one bunch. You solved add to and take from stories; these work the same way.
Key idea
Part + part = whole. If a part is missing, take the other part away from the whole.
Why it works
The two parts together are the whole group. However the group is split, the parts add up to the whole.
Worked example: Both parts known
There are 5 red balloons and 7 blue balloons. How many balloons are there?
- Both parts are known, so add.
- $5 + 7 = 12$
Answer: 12 balloons
Worked example: One part missing
There are 12 balloons. 5 are red. The rest are blue. How many are blue?
- The whole is 12. One part is 5.
- $5 + \square = 12$, so the missing part is 7.
- 7 are blue.
Answer: 7 blue balloons
For grown-ups: Sort a handful of mixed crayons into two colors. Say the parts and the whole: “5 red and 7 blue make 12.”