Abottle of the antiseptic hydrogen peroxide is labeled 3.0% (v/v). how many ml of hydrogen peroxide are in a 400 ml bottle of this solution

There are several ways of expressing concentration of solution. Few of them are listed below
1) mass percentage
2) volume percentage
3) Molarity
4) Normality
5) Molality

In most of the drugs, concentration is expressed either in terms of mass percentage or volume percentage. For, solid in liquid type systems, mass percentage is convenient way of expressing concentration, while for liquid in liquid type solutions, expressing concentration in terms of volume percentage is preferred. Present system is an example of liquid in liquid type solution 

Here, concentration of H2O2 is given antiseptic = 3.0 % v/v

It implies that, 3ml  H2O2 is present in 100 ml of solution

Thus, 400 ml of solution would contain 4 X 3 = 12 ml H2O2