# Double Double Division Algorithm

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 This page has been Tagged As Education and Tagged As Maths None of this is new, you can find it on the internet with the appropriate search. I know that. But having said that, I've never seen it laid out like this, and several people I've shown it to have been surprised by it and found it quite exciting.
I'm revisiting this, so I'd be interested to know where people "fall off the perch" and give up. I have my theories, but I'd like to get actual feedback.

For some of my readers, this will all be completely mysterious. But for others, part way through they will go "I recognise this!"

So I'd really like to hear from you ... if you recognise this, at which point did the penny drop? I'd also really like to know if you think this is obvious, and possibly even if you've seen it before.

Note: I don't need it explained to me - I know how it works! But if you'd like to show that you know how it works, then I'd be happy to hear that too.

So ...

Consider the following algorithm. I'll work through an example. I'll divide 450 by 23. We start by creating this table:

 A: . . . . . 0 B: 23 46 92 184 368 736 C: . . . . . 450

The middle row is doubling 23 until the answer is strictly larger than 450. Now suppose we have two columns, using a, b, c for quantities we know, and x and y for quantities we don't (yet) know, we complete the table by working leftwards:

Given this
situation:
 A: x a B: b 2b C: y c

If b >= c then
• set x = 2a
• copy c into y
If b < c then
• set x = 2a+1
• set y = c-b

Since 368 < 450, the next column is:

 2*0+1 A: . . . . 1 0 B: 23 46 92 184 368 736 C: . . . . 82 450 450-368

Since 184 >= 82, the next column is:

 2*1 A: . . . 2 1 0 B: 23 46 92 184 368 736 C: . . . 82 82 450 Copied

Since 92 >= 82, the next column is:

 2*2 A: . . 4 2 1 0 B: 23 46 92 184 368 736 C: . . 82 82 82 450 Copied

Since 46 < 82, the next column is:

 2*4+1 A: . 9 4 2 1 0 B: 23 46 92 184 368 736 C: . 36 82 82 82 450 82-46

Since 23 < 36, the next column is:

 2*9+1 A: 19 9 4 2 1 0 B: 23 46 92 184 368 736 C: 13 36 82 82 82 450 36-23

So I claim that 450 divided by 23 is 19 with remainder 13.

As I said, for some of you this will feel very familiar. At what stage did you get the connection (with the thing I'm carefully not mentioning)?

Have you seen this before?

Let me know.

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