A person saves 10% of his salary every month. If his salary increases by 12% and the expenditure increases by 10%, then what will be the change in his saving per month?

This question tests Basic Numeracy (Percentages), formally evaluated under the UPSC Civil Services Aptitude Test (CSAT) Paper-II framework, which tests candidates at a Class X mathematical standard.

Why Option B is correct: Applying standard percentage principles, we assume a baseline value of 100 to simplify the calculations:

• Initial State: Let the person's initial monthly salary be ₹100. Since he saves 10%, his initial saving is ₹10. Therefore, his initial expenditure is ₹100 - ₹10 = ₹90.
• Changed State: The salary increases by 12%, making the new salary ₹112. The expenditure increases by 10%. A 10% increase on the initial expenditure of ₹90 is ₹9, resulting in a new expenditure of ₹90 + ₹9 = ₹99.
• Final Calculation: The new saving equals the new salary minus the new expenditure (₹112 - ₹99 = ₹13). The absolute increase in saving is ₹13 - ₹10 = ₹3. Applying the standard formula for percentage change—[(New Value - Old Value) / Old Value] × 100—we get (3 / 10) × 100 = 30%. Thus, the saving increases by 30%, making Option B the correct answer.

Why the other options are incorrect:

• Option A (20% increase): Incorrect due to arbitrarily combining the percentages (e.g., averaging 12% and 10%) instead of applying them to the actual base values.
• Option C (03% decrease): Incorrect because it confuses the absolute numerical increase (3) with a percentage decrease, acting as a mathematical distractor.
• Option D (02% decrease): Incorrect as it mistakenly subtracts the percentage rates directly (10% expenditure - 12% salary = -2%), violating the mathematical rule that percentages on different bases cannot be directly added or subtracted.

Concluding Takeaway: Rule of Thumb: For percentage changes linking Income, Expenditure, and Savings, always anchor the initial salary at 100. Compute the independent absolute values for each component before calculating the final percentage change.