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9+8=17 17/2=8,5=~9 not 8

Since when is round(8.5) = 9 ? Not in my world

I'm Mathematician. So 8.5!=8 neither
Use Math.round pls

Usually in Math 8.4 is rounded to 8 and 8.5 is rounded to 9.

Well, apparently .NET by default uses the so-called banker's rounding:
http://stackoverflow.com/questions/977796/why-does-math-round2-5-return-2-instead-of-3-in-c
Which is fair enough, I guess, since it doesn't really make much difference.