what is volume of hydrogen peroxide means?

ini soalan yang cikgu Zuraifah tanya the other day.

listed here is the answer

Strength of hydrogen peroxide solution

It is customary to indicate the strength of a sample of hydrogen peroxide solution in terms of the 'volume of oxygen at NTP that one volume of hydrogen peroxide gives on heating'. When the grade of H₂O₂ in volumes is given, its percentage strength can be calculated as follows:
For example, we have a sample of H₂O₂ marked as 30 volume. Then, by definition of '30 volumes H₂O₂ means that 1 volume of this grade H₂O₂ gives 30 volumes of O₂ at NTP.
So, 1 litre of 30 volume H₂O₂ gives 30 litre of oxygen at NTP

From the equation, 2 H₂O₂ g 2H₂O + O₂ We find that, 1 mol of O₂ = 2 mol of H₂O₂
Thus, 30 volume H₂O₂ contains 2.678 mol per litre of this solution. Therefore the Molarity of 30 volume sample of H₂O₂ = 2.678
So, the strength of H₂O₂in the 30 volume sample = Molarity x Molar mass = 2.678 x 34 g L^-1 = 91.1g/L = 9.11%
'30 volume' sample of H₂O₂ actually contains 9.11% of H₂O₂