Also, 36-24 is 12, not 8, so you're missing 4 million there.
1st Year: Nov 2005 to Nov 2006
2nd Year: Nov 2006 to Nov 2007
3rd Year: Nov 2007 to Nov 2008
4th Year: Nov 2008 to Nov 2009
5th year: Nov 2009 to Nov 2010
6th Year: Nov 2010 to Nov 2011
7th year: Nov 2011 to Nov 2012
Let's say Microsoft does sell a little better over the last few years (though if you take bigger decreases than what I have used for arguements sake, it'll turn out the same), here would be 6th and 7th year sales, using PS2 numbers
6th Year: Roughly 25% decrease: 4.92 million
7th Year: Roughly 25% decrease: 3.69 million
Total After 7 Years (using PS2 sales trends): About 43 million
Using slightly lower PS2 sales (since we all agree 360 is no PS2, right?)
6th Year: Roughly 35% decrease: 4.26 million
7th Year: Roughly 45% decrease: 2.34 million
Total: About 41 million
Either way I still think 35 million is a good guess, since the PS3 will probably be taking some of the 360's market.
Edit: The 7th year would be after the release of the next gen consoles, according to your date (Nov. 2011), so ... yeah. I'll do ranges to make everyone happy. 
Ranges:
Wii: 120 million+ (depends on increased supply, lifespan, etc)
PS3: 30 million to 40 million
360: 35 million to 45 million
