Simple Interest – Infer principal from difference in interest rates: The difference between simple interest at 13% per annum and 12% per annum on the same sum for 1 year is Rs 110. Find the principal.

Introduction / Context:
At simple interest, interest is P * r * t. For the same P and t, the difference between two interest rates over 1 year directly equals P multiplied by the rate gap.

Given Data / Assumptions:

• Rate1 = 13% per annum
• Rate2 = 12% per annum
• Time = 1 year
• Difference in SI = Rs 110

Concept / Approach:
Difference = P * (0.13 - 0.12) * 1 = 0.01 * P. Hence P = Difference / 0.01.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• Rs 12000 / Rs 13000 / Rs 16000 / Rs 10000: produce differences other than Rs 110 when multiplied by 1%.

Common Pitfalls:

• Using percentage points incorrectly.
• Confusing difference in rate with ratio of rates.

Final Answer:
Rs 11000.