Pack-rate twist — buy 11 for ₹10, sell 10 for ₹11 A person buys 11 identical articles for ₹10 and sells them at a rate of 10 articles for ₹11. What is his gain percentage?

Introduction / Context:
This is another “x for ₹y” conversion problem. Converting both purchase and selling packs to per-unit prices allows a straightforward profit percentage calculation on cost.

Given Data / Assumptions:

• Buy 11 for ₹10 ⇒ CP per unit.
• Sell 10 for ₹11 ⇒ SP per unit.
• Uniform quality, identical articles.

Concept / Approach:
Compute CP per unit and SP per unit, then calculate gain% = (SP − CP)/CP * 100.

Step-by-Step Solution:
CP per article = 10/11.SP per article = 11/10.Gain% = ((11/10 − 10/11) / (10/11)) * 100.Compute: (121 − 100)/110 * 11/10 * 100 = (21/110) * (11/10) * 100 = 21%.

Verification / Alternative check:
Numerical check: CP ≈ ₹0.9091, SP = ₹1.10, margin ≈ ₹0.1909; 0.1909 / 0.9091 ≈ 0.21 = 21%.

Why Other Options Are Wrong:
20% and 22% are near but not exact; 19% and 1% are incorrect by larger margins.

Common Pitfalls:
Forgetting that profit percentage is on CP, not on SP, and attempting to average the pack rates directly.

Final Answer:
21%