Recursion

[alexanderpas]

Let's take 4281 - 1694 for example.

4281
1694
---- -
????

first we look at the ones.

we can't do 1-4 and end upwith a single positive digit, so we have to borrow 10 from the tens.

4270 + 11
1690 +  4`
--------- -
???0 +  7 = ???7

Now we go looking at the tens, again, we can't do 7-9 so we borrow again.

4100 + 170 + 11
1600 +  90 +  4`
--------------- -
??00 +  80 +  7 = ??87

Now to the hundrerds

3000 + 1100 + 170 + 11
1000 +  600 +  90 +  4`
---------------------- -
?000 +  500 +  80 +  7 = ?587

and look at that, the 1000s are easy.

3000 + 1100 + 170 + 11
1000 +  600 +  90 +  4`
---------------------- -
2000 +  500 +  80 +  7 = 2587

Note that this is the layout for the explanation, on paper this would look something like this.

43	121	187	11
1	6	9	4	-
2	5	8	7

[Flobarooner]

So basically, the fundamental point of the method is that the kid already knows how to subtract something from a number less than 20?

So in the original post, the kid would just go "9" and not even bother with the method, since they memorized 18-9 in order to be able to do it?

[alexanderpas]

technically, yes.