How can i determine the volume of the submerged part of the hydrometer when i'm given the density which is...?

it's density is 4.5 grams per cubic centimeter and then i'm also given the number 5 and 15 but i don't know what they're for- they're just something to do with the side graduated cylinder. HELP my worksheets due tomorrow and i have a test coming up on this soon too.