Mass-volume is a little confusing because you are calculating the amount of grams of solute (dissolved material) per mL of solvent (liquid the solute is dissolved in), as opposed to mass percent where you are comparing grams of solute to grams of solvent. this makes the terms a little stranger but the math is just as easy.
Basically, you are determining the percent of grams per mL of solvent - remember percent is always per 100, so your mass volume percent is going to be grams solute per 100 mL solvent.
Here is what we know:
We have 80 g. Solute (NaCl) and 4.30L Solvent (water).
Step 1) Convert your solvent from liters to mL.
The conversion factor is 1L = 1000 mL, so 4.30L x 1000 mL/1L = 4300mL
Step 2) We now know we have 80 g. Solute per 4300 mL solvent. How do we get that to g per 100 mL?
use the equation 80g / 4300 mL = X g / 100 mL and solve for X.
Multiply both sides of the equation above by 100 mL:
80g/430 mL = x g
0.186g per 100mL or .186 mass-volume %