CA Foundation · Quantitative Aptitude · Measures of Central Tendency and Dispersion
The mean monthly electricity bill of 8 flats in a Nagpur housing society is ₹1,250. When a ninth flat is included, the mean of all 9 flats becomes ₹1,300. What is the electricity bill of the ninth flat?
The ninth flat's bill is ₹1,700. The total for nine flats is 9 × 1,300 = ₹11,700, and the total for the first eight is 8 × 1,250 = ₹10,000. The difference of ₹1,700 is the new flat's bill.
- A₹1,450
- B₹1,700Correct
- C₹1,300
- D₹1,650
Explanation
Total for 8 flats = 8 × 1,250 = ₹10,000. Total for 9 flats = 9 × 1,300 = ₹11,700. Ninth bill = 11,700 − 10,000 = ₹1,700. Check: (10,000 + 1,700)/9 = 1,300. The option ₹1,450 results from adding only the rise of ₹50 × 9 = 450 wrongly to 1,000 or similar slips, and ignores the totals.
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