DS GMATPrep CD - If eleven consecutive integers are listed

[victorgsiu]

If eleven consecutive integers are listed from least to greatest, what is the average(arithmetic mean) of the eleven integers?

(1) The average of the first nine integers is 7.
(2) The average of the last nine integers is 9.

OA: D.


Could someone help out here?

[mikhsor]

Hi

(1) x is the first integer, then the average of the first nine integers is (x + (x+8))/2=7. Thus, x = 3. (x+(x+10))/2 = (3+13)/2 = 8

(2) x is the first integer, then the average of the last nine integers is ((x +2)+ (x+10))/2=9. Thus, x = 3. (x+(x+10))/2 = (3+13)/2 = 8

The answer is D

[victorgsiu]

Thanks for the post

[mkorrapati]

Average of ODD number of CONSECUTIVE integers is always the middle number. When you know average of first 9 numbers is 7, you know that fifth integer is 7. From this you can identify the 6th number to get average of eleven consecutive integers. You can apply same logic with average of last nine consecutive integers.

[RonPurewal]

Average of ODD number of CONSECUTIVE integers is always the middle number. When you know average of first 9 numbers is 7, you know that fifth integer is 7. From this you can identify the 6th number to get average of eleven consecutive integers. You can apply same logic with average of last nine consecutive integers.

very well done.