[math-fun] Improving on counting backward from 100 by sevens

I'd like to know the origin of this commonly-deployed cognitive test. It is a nice one I think—the first three stepping stones, 93, 86, and 79—have obscure factorizations, and it's only at 72 where I'm first able to be entirely confident that I haven't screwed it up, since 100-72 is "obviously" 28, which is 7*4. I'm weak at mental arithmetic generally speaking though. Are there any other similarly-mathy tests like this that people use? -- Thane Plambeck tplambeck@gmail.com http://counterwave.com/