The thing to remember about the density of a substance is that it's supposed to tell you the mass of exactly
In your case, the boulder is said to have a density of
You can thus use the density of the boulder as a conversion factor to calculate the volume of the
#800 color(red)(cancel(color(black)("g"))) * overbrace(color(blue)("1 cm"^3)/(8color(red)(cancel(color(black)("g")))))^("the given density = 8 g/"color(blue)("cm"^3)) = color(darkgreen)(ul(color(black)("100 cm"^3)))#
The answer is rounded to one significant figure.
So remember, when a problem provides you with the density of the substance, it's essentially giving you the mass of exactly