How to Guess Backwards From 1000 by Nine's

Students' ratings correlated very strongly with the end of semester ratings that students gave after taking the entire course. A psychologist had another set of participants count backwards from 1000 by nine's and guess what? Their ratings of those 10 second clips just as accurate. They, when they were like barely paying attention. Most people can't do that sober. Yeah, I'm not even sure one, counting 1000 backwards by nine's. Does that mean you subtract nine each time? So is it like nine hundred? Nine one? That's, I feel the writing test here. Give me a break.