% by volume is an expression of concentration that says the volume of solute (alcohol) per unit volume of solution (alcohol + water).
In this case, your unit of volume is mL so that 25 % v/v of alcohol solution means you have 25 mL of alcohol for every 100 mL of alcohol solution.
Let x be the volume of alcohol in 200 mL of 25 % v/v alcohol solution,
25 mL alcohol/100 mL alcohol solution = x mL alcohol/200 mL alcohol solution
x mL alcohol = (0.25)(200) = 50 mL
It is very easy!! :D
