That's true! But you don't want to pretend you know her weight exactly, when you don't. You just don't know whether she weighs exactly 120 lbs, or whether she's rounded her weight off to the nearest 10 pounds!
This is where the rules for significant figures can help you.
First: all non-zero digits in a number are significant figures.
Zeroes are significant when they come in between two other figures, like 305.
Leading zeroes, as in .06, are never significant.
Trailing zeroes are not significant unless the number contains a decimal point. So 600 has one significant figure, while 600.0 has 4 significant figures. After all, the person who says 600 might mean anything between 551 and 649. But if she says 600.0, she means she's measured it.
Another way for her to tell you how precisely she measured it is by underlining the last zero she measured. So if somebody writes 600, you don't know if she only measured the value to the nearest 100. But if she writes 600, you know she measured it to at least the nearest 10. 600 has two significant figures. 600 would have 3 significant figures.
Ms Slim weighed 120 lbs. How many significant figures were in that measurement?
Put your cursor in the black box below to check your answer.
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Now you've seen this example, click 'forward' and try it on your own until you get 10 right in a row!