What he ^ said!
Once you start to see numbers like this you will see things like why the difference of squares ‘work’.
Try for example doing 89 X 91 using the above method:
90 -1
90 +1
_____ x
90^2
-1 x 90 + 1 x 90 (=0)
-1 x +1 = -(1^2)
or
90^2 - 1^2
The whole point in mental calculation is making what looks difficult easy.
For that there are a lot of ‘tricks’.
The rest is plain addition and subtraction.
This is so true!
A lot of people find 124 - 89 difficult. Probably because of the double carry.
However; they can easily do 89-24 (=65).
The complement of 65 is 35 and that is your answer to 124 - 89.
Think in complements. If you see 54 try to also see its complement (46).
If you see 89 try to also see 11.
Subtracting 89 from 124 is the same as adding 11 (its complement) to 24.
As Robert said: