Okay, so I don't know if you guys have a special way (i.e. cross multiplication or something) but I would do this.
If 2 groomers can brush 8 horses in 3 hours, how many can one groomer brush in 3 hours? Just cut your horses in half and you get 4. In 3hours, one groomer can brush 4 horses. But how many can 3 groomers do in 3 hours? To get that, just multiply 3 (your number of groomers) by 4 (how many horses one groomer can do), and you get 12 (the number of horses your groomer can groom in 3 hours. So there you go, the answer is 3. It'll take three groomers 3 hours to brush 12 horses.
