Question

the word "proof" on alcoholic beverages is defined as twice the percentage by volume. How many ml of alcohol are in a one-liter bottle of 80-proof gin?

Answer 1

80 proof means 40% v/v alcohol.
% v/v means 40 g solute/100 mL solution.
So if 40 g alcohol are in 100 mL, how much is in 1000 mL?