Forgot what it's called but it's a math concept--my stats teacher showed us a similar problem with subtracting and adding from a hundred.

URK the concept is on the tip of my tongue and it's bothering me that I can't remember. Apparently you can't add and subtract from the two sides like that... is it carrying? continuous? Something like that. =_=;;; thanks, this is going to be killing me for the next week.