Books are purchased 11 for Rs 10 and sold 10 for Rs 11. Compute the profit percentage.

Introduction:
Different bundle rates for buying and selling should be converted to per-unit cost and selling price. Comparing those per-unit values leads to the exact gain percentage on cost.

Given Data / Assumptions:

• Buy 11 for Rs 10 ⇒ CP per unit = 10/11.
• Sell 10 for Rs 11 ⇒ SP per unit = 11/10.

Concept / Approach:
Profit per unit = SP - CP. Profit percentage = profit divided by CP times 100.

Step-by-Step Solution:
CP = 10/11 ≈ 0.9091SP = 11/10 = 1.1Profit per unit ≈ 1.1 - 0.9091 = 0.1909Profit% ≈ 0.1909 / 0.9091 * 100 = 21%

Verification / Alternative check:
Use fractions: Profit% = [(11/10) - (10/11)] / (10/11) * 100 = [(121 - 100)/110] / (10/11) * 100 = (21/110) * (11/10) * 100 = 21%.

Why Other Options Are Wrong:

• 10% and 11%: numerical approximations that ignore exact fractional comparison.
• 100%: misinterpretation of 10 and 11 counts.

Common Pitfalls:

• Comparing on the selling price base rather than cost price.

Final Answer:
21%