What is 1.999999 in 4 significant digit arithmetic?

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Say you are doing some calculations in 4 significant digit arithmetic and at some point you get something like 1.9999999. You need to use this number in further calculations, also in 4 significant digit arithmetic. Do you use 2.000 or 1.999?

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$$2.000$$

When calculating to $n$ significant figures, first write down the first $n+1$ digits of the number, and then round the first $n$ digits based on the value of the $n+1$th.

So I would write down $1.9999$ and then note that the $9$ at the end implies I round up, giving $2.000$ to $4$sf.