20% (w/w) means 20 g of H2O2 is present in 100 g of the solution.
Mass of solvent(water)=100g−20g=80g
Number of moles of H2O2=20 gmolar mass of H2O2=20 g34 g/mol=2034 mol
Molality of solution=Number of moles of H2O2Mass of solvent in kg
=2080×34×1000=7.35 m