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Follow up question: If it had ended at zero dollars, then the next day jumps to $5, what's the percentage change?

Follow up answer, it's never zero dollars.

Hum... We can know this today with the power of Math!

But seriously, changing to -$10 to -$20 is a +100% change. It's multiplication, this is how it works. From -$10 to $10 would be a -200% change.

Obviously, only the mainstream news reports finance numbers as a percentage of change. Anybody else is only concerned about ROI.

Well, NYSE stops at certain drop intervals, so percentage does also occur professionally:

https://en.wikipedia.org/wiki/Trading_curb

If I take 10$ to take some asset, and then later sell it for 20$ my return is 30$ and my investment was -10$, so my ROI comes out as -300% which betrays what's going on. So even with ROI, negative numbers throws a wrench in the machinerys typical assumptions.

I guess we'll find out tomorrow