A 15 L mixture has 20% alcohol and 80% water. After adding 3 L of water, what is the new percentage of alcohol in the mixture?

Introduction / Context:
This question examines percentage composition when a component (water) is added. The amount of alcohol remains constant while the total volume increases, changing the alcohol percentage.

Given Data / Assumptions:

• Total initial volume = 15 L.
• Alcohol = 20% of 15 L = 3 L; water = 12 L.
• Add 3 L of water; no alcohol is removed or added.

Concept / Approach:
After addition, alcohol quantity is unchanged; only the total volume increases. New alcohol percentage = (alcohol volume / new total volume) * 100.

Step-by-Step Solution:
Initial alcohol = 0.20 * 15 = 3 L. Add 3 L water ⇒ new total volume = 15 + 3 = 18 L. New alcohol % = (3 / 18) * 100 = (1/6) * 100 = 16 2/3%.

Verification / Alternative check:
Because we added only water, the alcohol fraction must decrease; 16 2/3% is lower than 20%, which is consistent.

Why Other Options Are Wrong:
17 and 18 1/2 do not match the exact fraction 3/18. 15% would require an even larger dilution.

Common Pitfalls:
Recalculating alcohol amount as if it changes. Only water is added, so alcohol volume stays 3 L; only the denominator changes.

Final Answer:
16 2 3