This question tests the application of rules for significant figures in addition and subtraction operations. According to the textbook context, for addition or subtraction, the final result should retain as many decimal places as are there in the number with the least decimal places. For part (a), the mass of the box (2.30 kg) has 2 decimal places, while the gold pieces converted to kg have more decimal places, so the sum must be rounded to 2 decimal places. For part (b), both gold piece masses have 2 decimal places, so their difference retains 2 decimal places.