Diluting a 15-L mixture by adding water: A 15-liter mixture contains 20% alcohol (and 80% water). If 3 liters of water are added, what is the new percentage of alcohol?

Introduction / Context:
Adding water does not change the absolute alcohol quantity, only the total volume. Therefore, the new percentage is alcohol mass divided by the increased total volume, expressed as a percentage.

Given Data / Assumptions:

• Initial alcohol = 20% of 15 L = 3 L.
• Added water = 3 L.
• New total volume = 18 L.

Concept / Approach:
New alcohol % = (unchanged alcohol) / (new total) * 100%. Substitute the values and reduce the fraction to a mixed number if needed.

Step-by-Step Solution:

Verification / Alternative check:
Decimal form: 0.1666… × 100% = 16.666…%, which matches 16 2/3%.

Why Other Options Are Wrong:
They assume changed alcohol amount or use incorrect total volume after addition.

Common Pitfalls:
Reducing alcohol amount by 20% of 3 L incorrectly; the alcohol content is unchanged by adding pure water.

Final Answer:
16 2/3%