One who can not acheive this on a rudimentary level is going to have a difficult time thinking about numbers in general.
SD
I'm with you Dave. I rarely set up algebraic equations to figure something out (sometimes I do), but I don't think I could flip numbers around in my head the way I do if I couldn't do it on an intuitive level. It was worth the pain.
OTOH, logic and reason are tools of the White Oppressor Man.