1000 g of a 50% (mass percentage) nitric acid solution is to be diluted to 20%(mass percentage) nitric acid solution. How many liters of water should be added to the starting solution? Please show me the detail answer

The dilution formula works as well for percent by mass as it does for molarity and normality.

%1 x grams1 = %2 x grams2.
Calculate grams 2, then subtract mass HNO3 in the 50% soln to see how much water to add.