Division with 2 digits simultaneously!

When mentally dividing, sometimes it makes sense - speed wise - to do 2 digits at the same time.
An example will make this clear, I hope.

100 / 37

If we were to divide by 40 as in 100 / 40 we immediately see 25 X 40 = 1000.
So divide 1000 by 40 = 25. Remainder is 0.
Then realise, since we are dividing by 37 instead of 40, the remainder is actually 3 x 25 = 75.
This remainder of 75 can be divided by 37. 75 / 37 = 2 with a remainder of 1.
Add the 2 to the 25 to get 27.
Realise this remainder of 1 is the starting point if we move 3 places to the right or a factor of 1,000.
We can cycle from the start.

So the answer must be: 2.7027027…

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I’m not sure I fully understand this, are you able to provide a couple more examples please?

Usually in division we generate 1 digit at a time.
However, there is no rule - if the situation presents itself - that you cannot do 2 digits a a time.

In this example, I start to work with 25 as the starting divisor.
Because 25X40 = 1000.
Because 40 is bigger than 37, the divisor is at least 25.
So I work with that and deal with the remainder in the next step.

Second step is that I calculate the remainder to be 75:

In other words:

(\frac{1000}{37}=25R75)

75 is bigger than 37, so 25 is too small.
Let’s correct this. Divide the remainder again by 37:

( \frac{75}{37} = 2R1 )

We add the 2 to the 25 to get 27:

( \frac{1000}{37} = 27R1 )

When dividing by 37, then 25R75 is the same as 27R1.

Stepwise: 25R75 = 26R38 = 27R1.

In the analogy with apples, this is what we do:
We have a pile of 1000 apples, that need to be divided over 37 baskets.

Step 1: We take 40 baskets and put 25 apples in each. The pile is now empty.

Step 2: We realize we only need 37 baskets so we remove 3 baskets, take the apples out and put them in a pile again. The pile now contains 75 apples.

Step3: We put 2 apples from the pile in each of the 37 baskets, leaving one apple left on the pile.

Now there are 27 apples in each basket and one left in the pile:

( \frac{1000}{37} = 27R1 )

Apart from Willem Bouman and and maybe Nodas, I know of nobody who memorized all the multiplication tables up to 100.
They would immediately take 27 as the first two digits.
Also because from experience they know that 37 X 27 = 32^2 - 5^2= 1024 - 25 = 999.

If you do this the traditional way you get:
Start with 100: 100\37. Guess 2 times. 2X37 = 74. 100 - 74 = 26.
260\37. Guess 7. 7x37 = 210+49 = 259.
260 - 259 = 1

You will end up with the same results. However, starting with 25 gives you much smaller numbers to work with.


The last part of the post is devoted to the fact that we started with 1 (if you move the decimal point) and ended with 1, so we work with a repeating string of digits and a factor 1000.


You ask for another example.
Let me think.
How about:

( \frac{1000}{47} )

Start with increasing the quotient to 50:
1000\50 = 20. Remainder is zero.

Lower the quotient from 50 to 47:

( \frac{1000}{50} = 20)
Therefore:
( \frac{1000}{47} = 20R60 )
(=3 baskets containing 20 apples each)

Divide 60 by 47:
( \frac{60}{47} = 1R13)
Add to 20R60=21R13:

( \frac{1000}{47} = 20R60 = 21R13 )

Let’s do another 2 digits:

Move the decimal point 2 places:
13 => 1300:

( \frac{1300}{50} = 26 )

( \frac{1300}{47} = 26R78 )
(3*26=78)

( \frac{78}{47} = 1R31 )

( \frac{1300}{47} = 26R72 = 27R31)

Another two:
( \frac{3100}{50} = 62 )

( \frac{3100}{47} = 62R(62*3) = 62R186 )

In my mind, I focus now in the 186 and a quotient of 50. 3 times. I subtract 150 from 186 to get 36 and add 3*3=9 to 36 = 45.

( \frac{3100}{47} = 62R186 = 65R45 )

( \frac{4500}{50} = 90 )

( \frac{4500}{47} = 90R270 =95R(270-250+3*5) = 95R35)

35 is more than half of 47, so round the 95 up to 96:

( \frac{1000}{47} \approx 21,276596 )

With 4 imho easy steps we have calculated 1000\47 to 8 significant places.

Observe:
We never multiplied by 47. We divided by 50 which is more like doubling the remainder and used the difference - 3 - as a factor to multiply by.

Btw, I am working on a post for 3 digit division. Should come up in the next couple of days.

Thanks Kinma, I get it now and have used it a few times. Look forward to the 3 digit division.

I like this method as it adds speed to division. I’ve mastered where the divisors are rounded up to a whole number eg 37, 48. What about the other way though eg 52, will we then get into negative remainders. The example I’m thinking off would be something like 1000 divided by 52, round that off to 1000 divided by 50 which is 20, then you have overshot the mark by 20 x 2, where do we go from there?

Indeed. From the negative remainder (of 40) there are two main avenues:

  1. Convert to positive. 20R-40 = 19R12. If the answer becomes one less, the remainder has to become 52 more. And continue like you normally would.
    In the analogy with the apples we take one apple out of each basket. This gives us 52 apples. We return the borrowed 40, so this gives us a remainder of 12.

  2. Keep on dividing, like this:
    Answer so far: 20R-40
    -40 => -400
    8 times. Add 400. Add. 16.
    Since we work with a negative remainder we need to subtract 8 from 200, so
    200R-400 => 192 R 16.
    Since the remainder is now positive again, continue like you normally would.
    16 => 160.
    3 times. Subtract 150 and subtract 6. R=4.
    Answer so far:
    19.23 R 4
    Shift decimal point: 40.
    zero times.
    19.230 R 40
    Shift decimal point: 400.

Now realize we have been here before (although negative).
So this is a repeating fraction. Subtract 230 from 999 to get 769:
Answer is therefore 19.230769 230769 …

Thanks Kinma, I prefer method 1 when dealing with negatives, it’s a bit more intuitive than the other method. I really like these shortcuts and they work well when you don’t have remainders after rounding off the divisors, in other words things like 800/52 becomes 800/50 which equals 40, it won’t work so well with 800/32 which becomes 800/30 and has a remainder. Thank you for sharing this.

800/32, of course has a much quicker way of calculating.
Because 8 is a factor of 32. :slight_smile:
Therefore: 800/32 = 100/4 = 25.

But I understand what you are trying to say and let’s work with that.
Let’s take 800/31.
Since we know that 800/32 = 25, we can take 25 as the first 2 digits, but let’s not do that. Instead, let’s overshoot by starting with 30. 30X31 = 930. Remainder is thus minus 130.
In order to get to a positive remainder we need 5 times 31 = 155.
So 30 R -130 = 25 R (-130+155) = 25 R 25.

A good way to train yourself on this is the following:
Mentally go like this:
30 R -130 = 29 R -99 = 28 R -68 = 27 R -37 = 26 R -6 = 25 R 25.
So just change the dividend one step at a time and modify the remainder accordingly.

That’s great Kinma, it’s wonderful to have all these different methods of dividing! Using your last approach, I’m imagining the most difficult situation would be something with decimals, so how would it go with something like 500 divided by 7.25? We have discussed other ways of dealing with that type of question but can we adapt what you have suggested above to this example?

500/7.25

First make a guess.
7 times 7 makes 49, so 70 times come to mind.
70*7.25 = 490+70/4 = 490+10+30/4 so answer should be around 70.
30/4 = 7.5
In other words, we are overshooting bij 7.5.

Answer is 70 R -7.5.
70 R -7.5 = 69 R -0.25 = 68 R 7

Now divide 7 by 7.25, and in order to show how to deal with negative remainders, let’s overshoot in all decimals.

Shift decimal point-
70
9 times 7 makes 63, so let’s take 10 times.
10*7 = 70 and 10 * 0.25 = 2.5.

However, when we add 10 to 680 R -2.5 we get 690 R -2.5, which was the middle step we just did in the previous range of: 70 R -7.5 = 69 R -0.25 = 68 R 7
Because: 69 R -.25 = 690 R -2.5
So this does not bring us forward.

Let’s work with 9 times and not overshoot in this step:
97.25 = 97 +9/4 = 63 + 9/4
Subtract 63 from 70 leaves 7.
9/4 = 2 1/4
Subtract from 7:
4 3/4

Answer so far: 68.9 R 4 3/4

Shift decimal point:
47.5
6 times would be my guess, so overshoot by taking 7 times.

77 = 49. subtract from 47.5 = -1.5.
7
1/4 = 1 3/4. subtract from -1.5 = -3 1/4

Answer so far: 68.97 R -3 1/4 = 68.96 R 4
As always, when you decrease a digit on the left part 7.25 gets added on the remainder part.

4 => 40
6 times in order to overshoot:

67 = 42 and 60.25 = 1.5
40 - 42 - 1.5 = -3.5

68.966 R -3.5 = 68.965 R 3 3/4

3 3/4 => 37.5
37.5 => overshoot by taking 6 times.
6 7 = 42 and 60.25 = 1.5

37.5 - 42 - 1.5 = -6
68.9656 R -6 = 68.9655 R 1.25

1.25 => 12.5.
2 times:
2*7.25 = 14.5. 12.5 - 14.5 = -2
So remainder is -2.
68.96552 R -2 = 68.96551 R 5.25

etc.

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Thanks Kinma, as usual well explained. We now have a number of strategies for dealing with division. Just a month or so ago, I only had one!