/sci/ - Science & Math

http://en.wikipedia.org/wiki/Rounding
CTRL-F [Rounding a number y to the nearest integer requires some tie-breaking rule]

i would round 9.35 to 9.4, and -9.35 to - 9.4
you round a 5 away from 0, because it is most likely to be accurate.
for example, if it was - 9.35429023... then to 2dp it is -9.35, but then by re-rounding it to 1 dp, -9.4 is closer to the origional, and this always works if you round away from 0.

yeh, u cant get anything other than a multiple of 9, with just + and -, if all you are using is multiples of 9, because multiples of 9 + or - other multiples of 9 always equals a multiple of 9 (or 0)
30, and 21 both are not multiples of 9, so you cannot make them.