Multiple choice

The average of $9$ numbers is $8$. What should be added as $10^{th}$ number to make the average $9$?

  1. $10$
  2. $72$
  3. $18$
  4. $90$
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C Correct answer
Explanation

Sum of 9 numbers = 9 * 8 = 72. Sum of 10 numbers = 10 * 9 = 90. The 10th number = 90 - 72 = 18.

AI explanation

The sum of the 9 numbers is 9 multiplied by 8, which equals 72. The desired sum for an average of 9 across 10 numbers is 10 multiplied by 9, which is 90. The difference between the new sum of 90 and the old sum of 72 is 18, so the 10th number must be 18.

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