My friend and I disagree about how to compute the difference between two drinks, one with 100 proof vodka and one with 80 proof. By volume the difference is 10% (40% vs 50% by volume), but in reality, isn't the one with 50% by volume 25% stronger than the one with 40% by volume?

In other words, how would this be answered:

The drink with 100 proof alcohol is ______ (% here) stronger than the drink with 80 proof alcohol.

Since "proof" is a measure of the liquor's alcohol content (proof number = 2 times the alcohol percentage), a 100 proof drink compared to an 80 proof drink is (100-80)/80 = 25% stronger.