Find number grams of the solute in a dissolution given its density and Molarity
Using 1L of solution, you have 16M of HNO3, each mole of which weighs 63g. Therefore, in 1L you'd have 63 X 16 = 1008g. Mass of 1L is given as 1420g from the density. 

%mass/mass = 1008/1420 = 70% HNO3