Difference is a comparison between two or more sets.
For example, if a set of 8 is compared with a set of 5, there is a difference of 3 objects between the sets.
This may be recorded as 5 + 3 = 8 or 8 – 5 = 3 where 3 is the difference.
In mental calculation it is often easier to regard a take away subtraction context as finding a difference. Adding on to solve the problem is frequently easier.
For example, 203 – 188 = ? is easier to solve as 188 + ? = 203.
Finding difference is contextually more difficult than taking away as the situation does not suggest a physical action to solve it.
You can read more about inverse operations.
