Here is how I would do 253 X 368:
In this case , since 25x36 is easy to do I split the numbers in 2:
25 | 3
36 | 8 X
The “|” is just there to show where I split the number.
Then I do the criss cross using 250, 3, 360 and 8 as the numbers.
So to drive the point home, I treat it as a 2x2, when it is actually a 3x3.
Also I do this from left to right.
So first step is 25 X 36 = 900. Actually, in my mind I call out 250 X 360 = 90,000.
Doing it this way also gives me the ballpark number.
Then comes the cross.
8 X 250 + 3 X 360
8 X 250 = 2,000 and
3 X 36 = 1,080
Total: 3,080
Add this to 90,000 gives 93,080.
Last step:
3 X 8 = 24
Add to 93,080 gives 92,104.
Then I check it with 9 or 11 proof.
On doing the above calculation, I realise I brute force the 11 proof.
1: I think “93,104 - 88,000” and immediately 5,104 comes up. No calculation. Maybe my mind does a quick 93-88, but the process is so quick that I don’t know for sure.
2: 5,104 - 4,400 = 704. Again; no calculation.
3: 704 - 660 = 44
4: 44 - 44= 0
This happens when I visualise the numbers. Only when I see both numbers, the result comes up.
Same with the numbers in the multiplication:
253 - 220 = 33
33 - 33 = 0
(actually I would stop here, since I now know that we will be multiplying with zero, so the end result needs to be zero. But for now let’s finish the 11 proof:)
368 - 330 = 36
36 - 33 = 3
3 x 0 = 0
Later today I realised this process does not always happen.
On a simple 45 - 18, nothing came up immediately.
When I raised the 18 to 20, then immediately 25 came up.
And then cascading, 27 came up as the answer.
For people who wonder why I am writing this; I am trying to find out when my brain immediately gives an answer and when not. See this thread and others.
Also because this got me thinking:
).