Both question 3 and 4 can be done in the same scaled way



3.  Given that a 1 solar mass black hole has a radius of 3 km
and that the black hole radius scales directly with the mass then
you know that a 10 solar mass black hole would have a radius of 30 km.

So, the mass of the earth is 1/330,000 the mass of the sun
hence its radius is 3km * (1/330,000) or about 9 millimeters

4.  density is mass divided by volume.  For the earth - we know
that 1 earth mass divided by the volume that corresponds to 1
earth radii is 5.5 grams per cubic centimeter.

In this problem its the mass of the sun into the radius of the earth
so the volume is identical and the density just scales with the mass.
so the density of this hypothetical object is 330,000 times more 
than the density of the earth or

330,000 x 5.5 which is about 1.5 million grams per cubic centimer
which is equivalent to the density of a typical white dwarf star.