ah. i forgot to tell another assumtion.

the standard deviation values of X and Y are 1%~20% of their mean values.

when the standard deviation value is bigger than that, the difference between the mean value of Z and (mean of X)/(mean of Y) become larger.

i don`t know what this assumtion is needed or not. but...i thought it is better to tell everything about my assumtions.

anyway thx!