Sept 29 - A class in the lab, and a class in Room 25

We only use the digital sum for 3 and 9...
http://www.youtube.com/watch?v=1ffvCtNZN9Q

But why does it work?
http://www.youtube.com/watch?v=ZLzzb8XkhCk

Homework:
Finish Pre-Test
Colour in all composite numbers on 1-200s chart
Test - Thursday