25.5% (w/w) means 25.5 g of H2O2 is present in 100 g of the solution.
Mass of solvent =100 g−25.5 g=74.5g
Number of moles of H2O2=25.5 gmolar mass of H2O2=25.5 g34 g/mol=25.534 mol
Molality of solution =number of moles of H2O2mass of solvent in kg
=25.574.5×34×1000=10.06 m