What can you say about density if volume increase and mass stays the same

Density = mass
              volume 

For the sake of explanation, let's pretend the mass is 2g and the volume is 1 ml

Density = 2 g = 2 g/ml
              1 ml

Now if we increase the volume, and leave the mass the same then mass is still 2g, but we can increase the volume to 2 ml 

Density = 2 g   = 1 g/ml
              2 ml

Therefore we can say that if volume increases and mass stays the same, then density will decrease.

Answer is to avoid experimental error