What is the best way to determine the volume of pure alcohol in a drink?

Question: What is the best way to determine the volume of pure alcohol in a drink?
  divide volume of alcohol by the patron's weight
  divide percentage of alcohol by volume of alcohol
  multiply volume of alcohol by the patron's weight
  multiply percentage of alcohol by volume of alcohol

Click here to know the Answer

multiply percentage of alcohol by volume of alcohol

Question: 4.0 ounces with 15% alcohol?
To calculate the answer:
15% and 4.0 ounces
15% = 15/100 = 0.15
0.15 X 4.0 ounces = 0.6 ounces of 100% alcohol

Answer: 0.6

Question: 1.0 ounce with 20% alcohol?
To calculate the answer:
20% and 1.0 ounce
20% = 20/100 = 0.20
0.20 X 1.0 ounce = 0.2 ounces of 100% alcohol

Answer: 0.2

Question: 20.0 ounce with 5% alcohol?
To calculate the answer:
5% and 20.0 ounces
5% = 5/100 = 0.05
0.05 X 20.0 ounces = 1.0 ounces of 100% alcohol

Answer: 1.0