I stumbled into this problem watching the greatest whisky review of all time – Horst Lüning’s review of Octomore 6.3. If you have time definitely watch the whole thing. Really. If you are time constrained, just start maybe around 9:20.
Anyway, at around 8:45 into the review, Horst adds a bit of water to dilute the drink from around 64% alcohol by volume to about 50%:
Here’s the question – if you had 100 ml of a 64% abv drink, how much water would you have to add so that the new mixture would be 50% abv?
Before answering the question, make sure to read about the partial molar property that comes into play when mixing water and alcohol.
I do not know the answer to the question, btw, but I did buy a bottle of the Octomore after seeing the review. Cheers anyway, though 🙂