To find the difference between two numbers, you can add up from the little one or subtract down from the big one: your result will be the same. When kids do diffies, they get lots of practice in additive differences—exactly what students do when they play race games—especially in the 2nd half of a race game: the race from your target number back down to zero.
This morning I finally got around to sorting through, spiffing up, and posting these PowerPoint animations that show how to do diffies. I’ve got a brief overview of diffies here. I’ll add more details later.
(Click below to download these how-to diffy animations in PowerPoint pptx)