I'm a father and a grandfather and have come up with a game for my offspring to play while we're on the road. When we see a license plate, the object is to be the first one to add all the numbers on it, and come up with THE one digit number that sums them up.
For example: ABC-787 = 7+8+7 = 22 = 2+2 = 4
Another example is 2932 = 2+9+3+2 = 16 = 1+6 = 7
Up 'til now, I've called it just plain "Numerology", but I'm sure that there's a math term for what we're doing, and I'd sure appreciate it if you could tell me what it is!