I assume that you're dealing with a 20% w/v solution, which would have 20 g of sodium hydroxide for every 100 mL of solution.
Since molarity is defined as moles of solute, in your case sodium hydroxide, divided by liters of solution, you need to use a sample of this solution to figure out how many moles of sodium hydroxide you would get per mole of solution.
To make the calculations easier, I'll use a 1-L sample. This sample would contain
#1000color(red)(cancel(color(black)("mL solution"))) * "20 g NaOH"/(100color(red)(cancel(color(black)("mL solution")))) = "200 g NaOH"#
Use sodium hydroxide's molar mass to determine how many moles you'd get in this many grams
#200color(red)(cancel(color(black)("g"))) * "1 mole"/(40.0color(red)(cancel(color(black)("g")))) = "5.0 moles NaOH"#
Since the sample had a volume of 1-L, its molarity will be
#C = n/V#
#C = "5.0 moles"/"1 L" = color(green)("5 M")#