Multiple choice

The average of 25 numbers is 20. The average of the first 12 numbers is 15 and that of the last 12 numbers is 18. Then, the 13th number is

  1. 100

  2. 101

  3. 104

  4. 103
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C Correct answer
Explanation

Sum of 25 = 25 * 20 = 500. Sum of first 12 = 12 * 15 = 180. Sum of last 12 = 12 * 18 = 216. 13th number = 500 - (180 + 216) = 500 - 396 = 104.

AI explanation

The total sum of all 25 numbers is 25 times 20, equaling 500. The sum of the first 12 numbers is 12 times 15, giving 180, and the sum of the last 12 numbers is 12 times 18, giving 216. The 13th number is found by subtracting these two sums from the overall total, so 500 minus 180 minus 216 leaves 104.

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