A man's average monthly expenditure for the first four months of a year is Rs. 231.25.\nFor the next five months, his average monthly expenditure is Rs. 22.75 more than this first average.\nHe spends a total of Rs. 605 during the last three months of the year.\nIf his annual income is Rs. 3500, what percentage of his annual income does he save in that year?

Introduction / Context:
This question combines average calculations, total expenditure over different periods, and percentage savings. You are given monthly averages for two blocks of months and a total amount for the remaining months, along with the man's annual income. The task is to compute how much of that income is saved, expressed as a percentage. This tests multi step reasoning, careful handling of averages, and percentage interpretation.

Given Data / Assumptions:

• Average monthly expenditure for first 4 months = Rs. 231.25.
• Average monthly expenditure for next 5 months is Rs. 22.75 more than 231.25.
• Total expenditure in last 3 months = Rs. 605.
• Annual income = Rs. 3500.
• There are 12 months in the year, divided into 4 + 5 + 3 months as provided.
• We must find (total savings / annual income) * 100.

Concept / Approach:
First, use the average for the first 4 months to find total expenditure in that block. Then, compute the new average for the next 5 months by adding Rs. 22.75 to the original average, and multiply by 5 to get total expenditure for those months. Add the known total for the last 3 months to obtain total annual expenditure. Finally, subtract this total expenditure from the annual income to find savings, and convert this to a percentage of the annual income.

Step-by-Step Solution:
Step 1: Total expenditure in the first 4 months = average * number of months = 231.25 * 4. Step 2: Compute 231.25 * 4 = (231 * 4) + (0.25 * 4) = 924 + 1 = Rs. 925. Step 3: Average in the next 5 months = 231.25 + 22.75 = Rs. 254. Step 4: Total expenditure in the next 5 months = 254 * 5 = Rs. 1270. Step 5: Total expenditure in the last 3 months is given as Rs. 605. Step 6: Total annual expenditure = 925 + 1270 + 605. Step 7: Compute 925 + 1270 = 2195; then 2195 + 605 = Rs. 2800. Step 8: Annual income = Rs. 3500, so total savings = 3500 − 2800 = Rs. 700. Step 9: Percentage of income saved = (savings / income) * 100 = (700 / 3500) * 100. Step 10: Simplify 700 / 3500 = 1 / 5 = 0.20, so savings percentage = 0.20 * 100 = 20%.

Verification / Alternative check:
We can quickly re check: total expenditure 2800 plus total savings 700 equals the income 3500, so the numbers are internally consistent. Also, saving 1/5 of the income corresponds to 20%, which matches our final calculation. No month is left out: 4 + 5 + 3 = 12 months are fully accounted for.

Why Other Options Are Wrong:
10%: Would mean savings of 350, giving total expenditure of 3150, which contradicts the computed 2800.

15%: Implies savings of 525 and expenditure of 2975, not matching any of the given blockwise totals.

25%: Implies savings of 875, leading to total expenditure 2625, which is inconsistent with 925 + 1270 + 605.

5%: Corresponds to savings of only 175, which conflicts even more strongly with the calculated expenditures.

Common Pitfalls:
A frequent mistake is to confuse the average increase for the next five months with an additional monthly amount on top of the yearly average, or to miscalculate the total for each block by simplifying 231.25 incorrectly. Another common error is forgetting to add all three blocks correctly before subtracting from income. Always compute each block total separately, check the month counts, and then add them up before working out the savings percentage.

Final Answer:
The man saves 20% of his annual income during the year.