Multiple choice

The average of 13 numbers is 80. The average of the first seven is 75 while that of the last seven is 83. Find the seventh number.

  1. 75

  2. 83

  3. 78

  4. 66

  5. 72
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D Correct answer
Explanation

Total sum = 13 * 80 = 1040. Sum of first 7 = 7 * 75 = 525. Sum of last 7 = 7 * 83 = 581. The 7th number is counted twice: (525 + 581) - 1040 = 1106 - 1040 = 66.

AI explanation

The sum of all 13 numbers is 13 multiplied by 80, which equals 1040. The sum of the first seven numbers is 7 multiplied by 75, which is 525, and the sum of the last seven numbers is 7 multiplied by 83, which is 581. Adding these two sums gives 1106, which counts the seventh number twice; subtracting the total sum of 1040 from 1106 leaves the seventh number as 66.

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