I started trying something a bit like that with a 0 at the front. Like you, I thought it might help, but I got met with the stony wall of silence....

I think she saw it as adding an illegal third number to the sum...

I quite like the idea of a sloping number line. I might be able to use that - might be able to rotate it various ways to do things like height above/below sea level.

I'll try it on Thursday.