Wikipedia has an excellent page on the matter - http://en.wikipedia.org/wiki/Significant_figures 480 only has two significant figures, because according to rule 4 on Wikipedia, trailing zeros in a number without a decimal point aren't significant. Therefore, in order to convert to one significant figure you will just have to round the number up to 500 - the trailing zeros still aren't significant, and you're creating another one. Two significant figures is easy because that's what it's already got, so just convert to scientific notation and be done with it. Three significant figures requires you to force the zero in 480 to become significant - adding a decimal place would do the trick. Four significant figures just means you need to add precision at the end. Look at rule 3 on the Wikipedia page. If you're measuring something, you won't write "1823.0000" unless you're using calipers accurate to ten-thousandths and you can honestly say that all those trailing zeros are still going to be zero if you measure them with something more precise. Think about it this way - if you measure an inch with a ruler, and it looks to be exactly an inch, you still can't write it any more precisely than 1.0 because that's all the resolution a ruler has. If you write it as 1.0000 and then measure it with the calipers from above, it'll come out to be something like 1.0321 - you can only add the zeros if can be read directly from the instrument used to measure them. If you have to guess at them, they aren't precise and therefore aren't significant.
