Use of significant figures in addition and subtraction My book mentions:


  The proper use of significant figures in addition and subtraction involves a comparison of only the absolute uncertainties of numbers. This means that only as many digits are retained to the right of the decimal point in the answer as the number with the fewest digits to right of the decimal


But I don't understand that why must the answer be rounded off after addition? Is the answer not accurate enough to be retained as it is ?
No. The answer isn't accurate enough. Say, for example I wish to add two numbers: $1.23$ and $2.367$.

I proceed like this, but I've no idea what the third digit after the decimal in $1.23$  is and so I represent it as $?$.

$$\begin{align*} 1 &amp;.23? \\ + 2 &amp;.367 \\ \hline 3 &amp;.59? \\ \hline  \end{align*}$$

This is why the end result is rounded off with the number of digits equal to the number having the fewest number of digits you initially set off with.