Question

Compare 51.5 hectograms to 51,500 decigrams. How do each of these values compare to a gram and which represents a larger mass?

Answer 1

These values when compared seem to be the same. They are equal. If we convert them to the same units, they results to the value which is:
 
51.5 Hectograms = 5100 Grams

51,500 Decigrams = 5100 Grams

Hope this answers the question. Have  a nice day.

Answer 2

51.5 Hectograms + 5100 Grams
51,500 Decigrams = 5100 Grams

They have same mass because they equal each other.