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The average of 15 numbers is 12. The first 8 average 11 and the last 8 average 14. The 8th number is:
A20
B18
C22
D24
Answer & Solution
Correct answer: A. 20
1. The total of all 15 numbers is 15 x 12 = 180.
2. The first 8 total 8 x 11 = 88.
3. The last 8 total 8 x 14 = 112.
4. Adding those two groups gives 88 + 112 = 200, but that counts 16 positions for only 15 numbers.
5. The 8th number is the one counted twice, so it equals 200 - 180 = 20.
_Source: NCERT Class 10 Maths, Ch 13 'Statistics', S13.2 Mean of Grouped Data_
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