Final answer:
To make an 80% alcohol solution, you need to add 50 mL of pure alcohol to 100 mL of 60% alcohol.
Step-by-step explanation:
The alligation alternate method can be used to solve this problem. To determine how many mL of pure alcohol is needed to make an 80% alcohol solution, we can set up the following diagram:
60% Alcohol Solution Pure Alcohol (100% Alcohol)
40 x 100
----------------- = -----------------
100 80
Cross multiplying and solving for x, we find that x = 50 mL. Therefore, 50 mL of pure alcohol must be added to the 100 mL of 60% alcohol to make an 80% alcohol solution.