Please help me find the pattern

212 , 106 , 135 , 67 , ___ , 47.5

212 - 106 = 106

135 - 106 = 29
135 - 67 = 68
what patterens do you see in that. if you don't understand i suggest going to google and typing in welcome to alabama. then click on the one that says "Welcome to Alabama's Free Live Online Homework Help Service. Click Here. This service offers free homework help from subject specialists..."