Multiple choice

If the average of 5 observations $x, x + 2, x + 4, x + 6$ and $x + 8$ is $11$, then what is the average of the last three observations?

  1. $13$
  2. $15$
  3. $45$
  4. $61$
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A Correct answer
Explanation

The average of x, x+2, x+4, x+6, x+8 is (5x + 20) / 5 = x + 4. Given x + 4 = 11, so x = 7. The last three observations are x+4, x+6, x+8, which are 11, 13, 15. Their average is (11+13+15)/3 = 39/3 = 13.

AI explanation

Using the average formula, the total sum of all five observations is 11 multiplied by 5, giving 55. Adding the expressions yields 5x plus 20, so setting this equal to 55 means 5x is 35 and x is 7. The last three observations are x plus 4, x plus 6, and x plus 8, which evaluate to 11, 13, and 15; their average is (11 plus 13 plus 15) divided by 3, equaling 13.

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