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Find their difference?

The numbers 23.982 and 3.4687 are both approximate and correct only to their last digits. Find their difference and state how many figures in the result are trustworthy.
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2 Answers

23.982-3.4687 = 20.513, which contains 5 significant figures.
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Attn: Since the result of addition or subtraction should be no more accurate than the least accurate one.

Comments

I'll take a liberty to answer on your question, Sun, and Robert J. will correct me if I'm wrong.
Because both numbers are with 5 significant figures, then result should be with 5 significant and trustworthy figures too. Only my question is, do we have to accompany the result with the relative error of the round-off?
Sun, what does your teacher expect? I would assume your teacher would have covered the definition of "trustworthy figures" to you at some point. Without knowing if this is a specific math problem (and no more), or if there's another part to this question (as with a statistical analysis or interpreting the results of a scientific study), it's difficult to answer your question.