Weird but serious question

Last Christmas, I was in a debate with my family regarding the alcohol percentage. For example, a can/bottle of premixed alcohol is 4.5%. When you drink half of that can/bottle, is it a till 4.5% or does it become 2.25%?. Im not too sure if I explained it correctly?

I can’t find the answer on Google to save my life, no matter how I phrase it.