# What's the word for addition and subtraction without borrowing or carrying over?

Is it regrouping? Upon googling it seems regrouping is borrowing or carrying over collectively. What's the word for not borrowing and carrying over? It's supposedly to train mental computation.

Example 1: When evaluating $21-9$, one can perform borrowing or do the following:

$$21-9$$ $$=10+11-9$$ $$=10+2$$ $$=12$$

Apparently the students who do this first build up mastery of addition and then subtraction facts like 11-9 so the idea is to teach students that they can reduce 21-9 to 11-9 to train mental computation instead of borrowing 10 from 20.

Example 2: For $12+9=21$, one can perform carrying over or do the following:

$$12+9$$ $$=12+8+1$$ $$=20+1$$ $$=21$$

The idea is that students are supposed to have mastered different ways to make 10 (1+9,2+8,3+7, etc) and then to make 20 (11+9,12+8,13+7,etc) so students would want to extract from 9 that which they can use to make 20 with 12, which is 8 to train mental computation instead of carrying over 10 from 2+9=11.

• Are you asking what this is called? It seems to be called "Bridging 10s" or "Making 10s" a la helpingwithmath.com/by_subject/addition/making-10-1oa6.htm – Opal E Aug 11 '17 at 19:41
• Mathematically, it's all just a direct application of the associative property. – rnrstopstraffic Aug 12 '17 at 4:55
• From your examples: decomposing and regrouping. Regarding the above comments, I favor the term friendly numbers (comes up in compensation strategies, too) and, just as associativity can be phrased as regrouping, so, too, can commutativity be phrased as reordering. – Benjamin Dickman Aug 13 '17 at 4:34
• @OpalE Thanks! Post as answer? Do you know where I can read more about this? Perhaps some debates or articles about which method is better or something? – BCLC Aug 16 '17 at 9:57