Go to http://nlvm.usu.edu 
Click on 'virtual library' 
Click on 6 - 8 (meaning 6-8th grades) 
Scroll down to Factor tree, click on it and follow the directions.  
Click on 'two' at the bottom. It teaches about the greatest common factor (GCF) and Least common multiple (LCM). 
There are many other good links to try also!!!!!!!!!!! 
 
Mr.Imler