The density of "NaCl"NaCl is "2.16 g/cm"^32.16 g/cm3, so I presume you are referring to a solution of "NaCl"NaCl with a density of "1.8 g/cm"^31.8 g/cm3.
An "NaCl"NaCl solution with a density of 1.8 g/mL contains 24 g "NaCl"NaCl in 100 g of solution.
∴ For 20 g of your solution,
"Mass of NaCl" = 20 color(red)(cancel(color(black)("g solution"))) × "24 g NaCl"/(100 color(red)(cancel(color(black)("g solution")))) = "4.8 g NaCl"
"Volume" = 20 color(red)(cancel(color(black)("g solution"))) × "1 mL solution"/(1.8 color(red)(cancel(color(black)("g solution")))) = "11.1 mL"
When you add this solution to 500 mL of water, you will have 4.8 g "NaCl" in 511.1 mL of solution.
∴ "Percent (m/v)" = "4.8 g"/"511.1 mL" × 100 % = "0.94 % NaCl"
