When you add and subtract with sig figs, you add the two numbers first:
24.545+307.3=331.845
You then count the number of sig figs in each original number, and the answer will be the one with the least number of sig figs. In this problem, 307.3 has the least amount of sig figs, which is 4.
Round the answer so that it has 4 sig figs:
331.845 --> 331.8
The answer is 331.8