The unsung hero. A very large part of mental calculation is addition and subtraction.
Any weakness here is death. Alternately, Every tiny improvement here reaps benefits with a nice multiplier effect.
I often stumble when carrying, Having the idea that the complement of 7 is 3 and 47 is 53 at the forefront of your thoughts process removes a chain of reasoning that seems to take forever when you do it. I regularly find myself mentally carrying the one, reducing, adding the 10’s to the 1’s and then subtracting. This takes a bunch of time, is distracting from the larger calculation, and is error prone as heck. If I could choose to unwire something explicitly it would be how I learned to add and subtract at an early age. Observing the complement seems like a far more natural thing than long step addition and subtraction.
Maybe it is just the soroban practice turning my brain to mush but if you don’t have this nailed I expect that it is an opportunity for big gains at low cost.
It was this video that popped the idea into my head…
The missing video from this series is the subtraction with complements but if you think about additions in this fashion then drill subtraction in the same fashion then it should be reasonably obvious. We all know this but if thinking about it in the right way when calculating doesn’t come naturally then like me you may be wasting a bunch of time and energy. That and a bunch of practice to drill the technique. till I stop doing it the hard way.