Density equals mass over volume; so volume equals mass divided by density, and mass equals density times volume. You want to find the mass of a given volume of water at two temperatures. At 4ºC water has a density of (essentially) 1.0 g/mL. At 99ºC, density is about 0.96 g/mL. So if the volumes are the same, the cold water would have more mass for that volume.