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  1. The average of 15 numbers is 7. If the average of the first 8 numbers be 6.5 and the average of last 8 numbers be 9.5, then the middle number is
    1. 20
    2. 21
    3. 23
    4. 18
Correct Option: C

Here , Sum of 15 numbers = 15 × 7
Sum of the first 8 numbers = 8 × 6.5
and Sum of last 8 numbers = 8 × 9.5
The middle number = ( Sum of last 8 numbers + Sum of the first 8 numbers ) - Sum of 15 numbers
The middle number = 8 × 6.5 + 8 × 9.5 – 15 × 7
The middle number = 52 + 76 – 105
The middle number = 128 – 105 = 23



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