Discussion
The average of 21 results is 15. The average of first 10 is 12 and the last 10 is 14. Find the 11th result?
Answer: D
The total of all 21 results is the average times the number of results, and the 11th result is what is left after taking out the first ten and the last ten.
\[ 21 \times 15 = 315,\quad 10 \times 12 = 120 \] \[ 10 \times 14 = 140 \]
\[ \text{11th result} = 315 - 120 - 140 = 55 \]
The values 50, 52 and 54 would come from arithmetic slips in the sums, for example forgetting that the middle result belongs to neither group. The answer is 55, option D.
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