Rate-to-quantity conversion: A shopkeeper sells eggs at 20 for a rupee and gains 30%. How many eggs did he buy for a rupee (i.e., what was his purchase rate)?

Introduction / Context:
When selling rate and profit% are known, you can back-compute the cost per unit and then infer how many units were bought per rupee.

Given Data / Assumptions:

• Selling rate: 20 eggs for ₹ 1 ⇒ SP per egg = ₹ 1/20.
• Profit = 30% on cost.

Concept / Approach:
SP = CP * (1 + profit%). Hence CP per egg = SP / 1.30. Convert the CP per egg back to “eggs per rupee.”

Step-by-Step Solution:
CP per egg = (1/20) / 1.30 = 1/26 rupeeEggs per rupee at cost = 1 ÷ (1/26) = 26

Verification / Alternative check:
Cost for 26 eggs = ₹ 1; selling 26 eggs at 20 per rupee gives revenue = 26/20 = ₹ 1.30, which is 30% profit on ₹ 1.

Why Other Options Are Wrong:

• 30 / 35 / 40: do not match the 30% profit when reconciled with SP = 1/20 per egg.

Common Pitfalls:

• Mistaking “20 for ₹ 1” as cost instead of selling rate.

Final Answer:
26