Using the rules for significant figures, what do you get when you add 24.545 and 307.3?

The answer depends on the number of significant figures required for the question. This would need to be stated.
There are different ways of stating the level of accuracy

We add the numbers as usual to get #331.845# and then round it as required.

#color(red)(331.845)# has 6 significant figures and 3 decimal places.

#color(red)(331.85)# has 5 significant figures and 2 decimal places.

#color(red)(331.8)# has 4 significant figures and 1 decimal place.

#color(red)(332)# has 3 significant figures and is rounded to the nearest whole number.

#color(red)(330)# has 2 significant figures and is rounded to the nearest 10.

#color(red)(300)# has 1 significant figure and is rounded to the nearest 100.

When adding or subtracting measurements (numbers), the result must have the same number of decimal places as the number in the problem with the fewest decimal places.

#24.545 + 307.3=331.845#

However, the answer must be given to the correct number of decimal places, in this case #1# in #307.3#.

So the result of the addition of #24.545# and #307.3# should be reported as #331.8#.

For more information and practice, go to this website:
http://www.chemteam.info/SigFigs/SigFigMath-AddSub.html