Design discharge composition: If the ratio of storm drainage to sanitary sewage is 20:1, what percentage of the total design discharge flows during non-monsoon (sewage only) periods?

Introduction / Context:
Combined or parallel drainage systems often consider design discharges that include large stormwater components seasonally. During dry weather (non-monsoon), only the sanitary sewage portion flows. This question evaluates proportional reasoning applied to design ratios.

Given Data / Assumptions:

• Ratio of storm drainage to sewage = 20 : 1.
• Total design discharge during monsoon = storm + sewage = 20 + 1 = 21 parts.
• Non-monsoon discharge consists only of the sewage component.

Concept / Approach:
Compute the fraction of the total design discharge represented by sewage alone, then convert to percentage.

Step-by-Step Solution:
1) Total design parts = 20 (storm) + 1 (sewage) = 21.2) Sewage-only fraction in dry weather = 1 / 21.3) Convert to percentage: (1 / 21) * 100 ≈ 4.76% ≈ 5%.

Verification / Alternative check:
Check arithmetic: 21 * 5% = 1.05 parts, very close to the 1 part sewage component; rounding explains the small difference.

Why Other Options Are Wrong:
10%, 15%, 20%, 25%: These overestimate the sewage-only share relative to the given 20:1 design ratio.

Common Pitfalls:

• Confusing the ratio 20:1 with 1:20 or misallocating which part is sewage.
• Forgetting to convert fraction to a percentage.

Final Answer:
5%