The mass of a cm3(cube) stone is 180.
a) calculate its density.
b) the stone is broken into two peaces. what is the density of each piece.
c) the smaller piece weights 80g. calculate its volume.

 Density = (mass) / (volume), no matter how large or small the sample is.

We can't calculate the density, because you left out the number for the volume.
Also, you didn't tell us the unit for the mass of 180.

a).  If the mass is 180 grams, then the density is 

                               (180 gm) / (volume) .

b).  No matter how many pieces you crush it into, and
no matter how large or small a piece is, its density is
the same.  (I just wish we knew what the density really is.)

c).  A piece may have 80 grams of mass.  It doesn't "weigh" 80 grams.

Since the density of the whole rock is  (180 gm) / (volume),
the density of any piece of it is  (180 gm) / (volume).

Multiply each side by (volume):   (Density) x (volume) = 180 gm

Divide each side by (density):        Volume = (180 gm) / (density)

We can't calculate the volume of an 80-gm piece, because
we don't know the density.  (That's because you left the volume
out of the question.)