@ShimizuA @empressnyannyanau Well, if you're an accounting student you'll know that most of the accounts we do have a lot of 0s at the end. Like "X bought $20000 worth of computers" and whatnot. And often times when you're writing them down you mess up and add an extra 0 or miss a 0 cause it's the same number repeatedly and you don't even realize it cause it looks the same. So when you add it up you'll realize that it doesn't match with the other number if there are two values that should be similar. Like for example Debit and Credit, or Assets and Liability + Owner's Equity. Those numbers should turn up the same but if you find that they're not and most of your calculations end with lots of zeros...you can just subtract the smaller number from the bigger number, then you'll find how much you've calculated wrong.
If you then divide it by 9 you'll get the original number that you should've inputted or the wrong number you've inputted (depends on if you've add an extra 0 by mistake or forgot a 0).
Example:
Your total asset is 100000.
Liability is 20000 and Owner's Equity is 80000.
So in one column you've got 100000 which is the asset.
In the other column you've made a mistake and added 2000 (instead of 20000) + 80000. Which is 82000.
So now the two columns don't match up. You've got 100000 on one side and 82000 on another.
So you subtract the bigger value with the smaller value, 100000 - 82000 = 18000.
Now divide that by 9 and you get 2000.
That's the number you got wrong.
So got back to your chart and look at all the values and see where you wrote 2000. Now you don't have to go through all of them one by one while counting 0s and checking to understand where you did wrong.
This is good for when there are a lot of calculations being done. Imagine you have 20 different accounts that you've added up and found that the values don't match up. Now you have to go through each one of them one by one and check with your textbook to see where you inputted the wrong number.
But if you use this method you can just easily find the number you've got wrong.
Also, it works the other way around too.
If instead of missing a 0 you've added an extra one by mistake. Like 200000 (instead of 20000) + 80000 = 280000.
If you subtract 100000 (the asset) from it you get 180000. Now divide that by 9 you get 20000. The original number you were supposed to input.
Now, since I am not from an English speaking country so I'm not familiar with the respective English terms used here so it may or may not be a little difficult for you to understand.
But basically, it's not a super practical method since it only works when all the numbers end with multiple 0s and assuming the mistake you made is with that 0. But in your school and college life at least all your accounts will end with multiple 0s. So it's a really useful hack for students. But that's just what it is, a hack. Not very practical but it's got its uses in specific situations. 90% of the time when you've made a mistake in calculation it's usually you just adding an extra 0 or forgetting one. So it's not sure to catch the mistake since if you've made a mistake you don't really know what type of mistake you've made, it could be a completely different mistake. But since it's a commonly done mistake it's a useful hack.