I haven't used this idea since high school, really, but it comes up now and then, so I looked it up in Wikipedia
. 100 has one significant figure, as do 20 and 23 and .0001, the article says. The number .00200, however, has three significant figures. The number 1.234 has 4 significant figures. Digits beyond accurate measurement don't count as significant. There is ambiguity, however, in whether 100 feet really has just one significant figure. It may be that you have measured it to the nearest foot, in which case it really has three significant figures.
The real importance of significant figures comes in doing arithmetic. If you run 100 yards in 11.71 seconds, and the 100 has three significant figures, then the speed should be written with three significant figures as 8.54 yards per second, not as 8.53970965 yards per second.
Labels: math, science