well, rounding is only done in favor of less writing down (are you going to write every decimal? have fun with Pi) but that doesn't mean you can round off and calculate on with the rounded off numbers. You always continue with the non rounded numbers. I have been punished for not doing this in school (our teacher sometimes was slightly sadistic in making math tests like these). In maths acutally 2 things you need to know: 1) what is a function? (and how does it work and hwat can you do with it) 2)every numeral representation of a number is a range unless otherwise defined or reasoned.