10 volume H₂O₂ means 1 L of H₂O₂ gives 10 L O₂ at S.T.P according to equation
2 mole H₂O₂ give 1 mol [22.4L O₂ at S.T.P]
0.89 mol H₂O₂ is present in 1 L solution
Assuming density of solution as
1 g ML^-1 same as density of water
1 L solution = 1000 g mass of H₂O₂ = 0.89  x 34
= 30.26 g
Percent strength of H₂O₂
$\begin{array}{l}=\frac{\text{ Mass of }{\mathrm{H}}_2{\mathrm{O}}_2\left(\mathrm{g}\right)}{\text{ Mass of solution }\left(\mathrm{g}\right)}\times 100\\ =\frac{30.26}{1000}\times 100=3.026\mathrm{\%}\end{array}$