Well if one was 40% and one was 50%, the mixture of two equal amounts would make 45%. Right?
Depends on how much you mixed of each.
If you had 1 liter of 40 percent vodka, common sense would tell you there's .40 liters of vodka.
Then if you had 1 liter of 50 percent Crown Royal, 1/2 liter is alcohol.
Mix the two.
Now you have 2 liters of vodka, with .90 liters of alcohol. Divide to simplify, and yes, for every 1 liter there is .45 liters of alcohol. 45 percent.