Density is defined as mass per unit of volume.
Mathematically, the relationship between density, mass, and volume can be written like this
In your case, you know that ether has a density of 0.71 g/mL. This means that every mL of ether has a mass of 0.71 g. Likewise, you can say that every 0.71 g of ehter occupy a volume of 1 mL.
Thus, the 250-g sample will have a volume of
Rounded to two sig figs, the answer will be
Now for the mass of the urine sample. The density of urine in a patient with kidney malfunction is known to be 1.001 g/mL.
This means that every mL of urine has a mass of 1.001 g. Since you have 20 mL in your sample, you'd get
Rounded to one sig fig, the number of sig figs you gave for the volume of the sample, the answer will be