Multiple choice

The average of 17 numbers is 45. The average of the first 9 numbers is 51 and that of the last 9 numbers is 36. Find the 9th number.

  1. 14

  2. 20

  3. 18

  4. 22
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C Correct answer
Explanation

Sum of all 17 numbers = 17×45 = 765. Sum of first 9 numbers = 9×51 = 459. Sum of last 9 numbers = 9×36 = 324. When we add the sums of first 9 and last 9, the 9th number is counted twice. So 459+324-9th number = 765, giving 783-9th number = 765, so 9th number = 18.

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