Actually whether 2.5 mL is a correct answer depends on what kind of concentration you are given - is it 50% (and 10%) w/w, or w/v? Or even v/v?
In the last case things is a bit easier - assume amount to be added is V (unknown). Then final volume is 10+V, you need 10% of 10+V to be pure ethanol, and you know amount of pure ethanol added is 50% of V. Can be written as an equation in one unknown, simple algebra.
Not that the difference will be huge in other cases, but definitely measurable.