So here students have selected the number
line. so they can drag it onto the display,
make it a little bigger if they want. Here
we want to set the minimum at zero, the maximum
at 6 so we can see five point nine, and our
interval by one-tenth. That's the smallest
interval we can use. So we can build out line.
So here is one detriment to using this tool
- you cannot see the whole number line all
together. This doesn't, just shrinks the space
it takes, not unfortunately the number line
to see it all. So students using this tool
might say, well, I want two decimals, two
decimal numbers that sum to five decimal nine.
Well we could use subtraction. Or they could
say if I go down a whole, to here. So 1 whole
subtracted from five and nine tenths is four
and nine tenths. so my two numbers could be
four decimal nine. Oops. [not sure why I'm
not getting that] There we go. Four decimal
nine plus 1 whole equals five decimal nine.
And we know they don't need the whole [word]
written in there. Another possibility might
be to set this to five and say well if I start
with five how much would I need to further?
I would need nine tenths. So five plus nine
tenths equals five decimal nine. And then
we hope they would write that as five plus
zero decimal nine equals five point nine.
Some students may write it like this. And
that opens up a good discussion whether these
are equivalent statements, in fact all three
of them, or if we need to put the zero, or
if we should put the zero; what value does
this zero have? Does it help our understanding?
Is it necessary? Or is it just good practice?
Going back up here I could say well I know,
let's pick another number. So for example
lets pick two. Well from two to three is a whole.   [move this off] 
 And from three to four is
another whole. So let's see the other whole.  So let's see - should write my thinking down as I'm doing this.  Two plus a whole plus another whole gets me to four.  Plus another whole gets me to five.  I thought I could add that but I can't, now that I'm off there.  So let's clear that.  So let's start that again.  Two plus a whole plus another whole plus got me to four, plus another whole will get me to five plus I now know from this one here I need nine more tenths.  Plus zero decimal nine.  So this number line, this virtual number line isn't the best because students would need to hold decimal numbers in their heads.  But it does allow for some manipulation and possibly that is, won't bother some of the students.  So here are some different combinations.  Students could be invited to see if this combination is similar or different to anyone else's.  And how I might write this differently.  So could I make this 3 plus two plus zero point nine?  Or three plus two and nine tenths?  Lots of variations possible again.  And its getting kids to see which ones are the same and which are different by talking to other students in the class.
