Firstly, several concepts are clarified: the gold density is19.3g/cm3; The density of silver is10.5g/cm3. So when this thing is pure gold, the volume V should be: 1000A/ 19.3 (cubic centimeter) = A/ 19.3 (cubic decimeter), and when this thing is pure silver, the volume V should be: A/ 10.5 (cubic centimeter).

When the gold content is 0.9, that is, 90% is gold and 1% is silver, the volume at this time is: v = 0.9a/19.3+0.1a/10.5 (cubic decimeter), because the higher the gold content, the more the volume. Then the volume v of this Dongdong is greater than a/ 19.3 (cubic decimeter) and less than or equal to 0.9a/19.3+0.1a/10.5 (cubic decimeter).