The density of #"NaCl"# is #"2.16 g/cm"^3#, so I presume you are referring to a solution of #"NaCl"# with a density of #"1.8 g/cm"^3#.
An #"NaCl"# solution with a density of 1.8 g/mL contains 24 g #"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"#
