Technically, they can.
Density is a function of mass as well as volume, so one volume of water V can have the same density as another volume of water V' if the difference in the their mass is just right. The change in mass can be accomplished by adjusting yet another variable outside of the equation
density = mass / volume,
namely temperature.
Research shows the density of water can vary from around 0.9957 g/cm³ at +30°C to a maximum of 1.0000 g/cm³ at 4°C, to 0.9839 g/cm³ at -30°C.
(source: hypertextbook.com /facts /2007 /AllenMa .shtml)
I think the answer they're looking for might be "no", but the author of this question really should have clarified that all other characteristics of the two volumes of water are kept the same.
