How to solve...

Report each answer to the correct number of significant figures:

$23\mathrm{.}9+1\mathrm{.}22,7\mathrm{.}21+7\mathrm{.}9$

$3\mathrm{.}90\mathrm{.}0560,100\mathrm{.}4-22$

$1\mathrm{.}4731,13\mathrm{.}993+0\mathrm{.}22$

A dry, empty beaker weighs $55\mathrm{.}392$ grams. You add some water to the beaker and it now weighs $68\mathrm{.}44$ grams. What is the mass of the water using the correct number of significant figures?