Amitabh spends on average Rs 4200 per month from January to June. His expenditure in January is Rs 1200 and in July it is Rs 1500. What is his average monthly expenditure from February to July?

Introduction / Context:
This question is about recomputing an average when you change the time period. You are given the average for one block of months and specific monthly values for some months. Then you are asked to find the average over a different block of months that partly overlaps with the first.

Given Data / Assumptions:
- From January to June (6 months) the average monthly expenditure is Rs 4200.
- Expenditure in January is Rs 1200.
- Expenditure in July is Rs 1500.
- We must find the average expenditure from February to July, which is 6 months again (February, March, April, May, June and July).

Concept / Approach:
First, we convert the average for January to June into the total expenditure for that period. Then we subtract the amount spent in January to get the total for February to June. After that we add the expenditure in July to obtain the total for February to July. Finally, we divide this total by 6, because this second period also has six months.

Step-by-Step Solution:
Step 1: Total expenditure from January to June = average * number of months = 4200 * 6. Step 2: Compute 4200 * 6 = 25200 rupees. Step 3: Expenditure from February to June = 25200 - 1200 = 24000 rupees. Step 4: Add July expenditure to get expenditure from February to July: 24000 + 1500 = 25500 rupees. Step 5: Number of months from February to July is 6, so the new average is 25500 / 6. Step 6: Compute 25500 / 6 = 4250 rupees per month.

Verification / Alternative check:
We can check that the total from February to July, 25500, divided by 6 indeed gives 4250. This is consistent and within a realistic range given that several months must have higher expenditure to counterbalance the very low January value of Rs 1200.

Why Other Options Are Wrong:
- Option Rs 4520: This would imply a total of 27120 for six months, which exceeds the possible total given the average for January to June.
- Option Rs 4060: This total (24360) is too low because February to June plus July must exceed the January to June total due to replacing January 1200 with July 1500.
- Option None of these: This is incorrect because Rs 4250 matches the correct calculation.

Common Pitfalls:
One common error is to add January and July directly or to mistakenly average January to July instead of February to July. Another is forgetting that the number of months in the new period is still six. Always write out which months you are including and carefully compute totals for the exact range specified.

Final Answer:
The average monthly expenditure from February to July is Rs 4250.