Recall one of the rules for significant figures
A final zero or trailing zeros in the decimal portion ONLY are significant
24.00 mg has four significant figures
2,4, 0 and 0
24.00 mg when converted to grams is 0.024 g
0.024 only has 2 significant figures, therefore we need to add 2 more zeroes at the end of 4
0.024 g → 0.02400 g