A 20% alcohol solution: 15 litres of this mixture receives 3 litres of water. What is the new percentage strength of alcohol?

Introduction / Context:
Dilution problems preserve the amount of solute (alcohol) while changing the total volume by adding solvent (water). The resulting concentration is solute amount divided by the new total volume, expressed as a percentage.

Given Data / Assumptions:

• Initial mixture = 15 L at 20% alcohol.
• Added water = 3 L.
• No alcohol is lost or gained during mixing.

Concept / Approach:
Compute the absolute amount of alcohol initially, then divide by the new total volume after adding water. Finally convert the fraction to a percentage.

Step-by-Step Solution:

Verification / Alternative check:
If 16 2/3% of 18 L is alcohol, quantity = (1/6)*18 = 3 L, consistent with the conserved alcohol amount.

Why Other Options Are Wrong:
12 1/2% corresponds to adding more water or miscounting initial alcohol; 24% is impossible since adding pure water cannot raise concentration; 16% is due to rounding down.

Common Pitfalls:
Treating percent changes additively or forgetting to first compute the absolute solute before re-percentage on the new volume.

Final Answer:
16 2/3%