A list of 11 positive integers has a mean of 10, a median of 9, and a unique mode of 8. What is the

Poftethef9t

Poftethef9t

Answered question

2022-06-06

A list of 11 positive integers has a mean of 10, a median of 9, and a unique mode of 8. What is the largest possible value of an integer in the list?
From the information, I got the following information:
11 integers with a mean of 10 means a total must be 110.
A unique mode means that there must be at least two 8's.
if there are two 8's, and the median is 9, there must be at least two numbers greater than 9 also.

Answer & Explanation

Ethen Valentine

Ethen Valentine

Beginner2022-06-07Added 15 answers

Taking the numbers to be arranged smallest to largest, we have:
x 1 + x 2 + x 3 + x 4 + x 5 + 9 + x 7 + x 8 + x 9 + x 10 + x 11 = 110
We want to make x 11 as large as possible, while keeping everything else positive, and making sure 8 is the unique mode. This means making the sum of x 1 through x 10 as small as possible, while respecting the same constraints.
As a first case, we try using exactly two 8's, so every other number can only occur once. This gives us:
1 + 2 + 3 + 8 + 8 + 9 + 10 + 11 + 12 + 13 + x 11 = 110
or x 11 = 33
As a second case, let's try three 8's, which allows us to use each other number twice:
1 + 1 + 8 + 8 + 8 + 9 + 9 + 10 + 10 + 11 + x 11 = 110
or x 11 = 35
That seemed to help, so let's try four 8's:
1 + 8 + 8 + 8 + 8 + 9 + 9 + 9 + 10 + 10 + x 11 = 110
or x 11 = 30
Finally, with five 8's, we have:
8 + 8 + 8 + 8 + 8 + 9 + 9 + 9 + 9 + 10 + x 11 = 110
or x 11 = 24
This exhausts the cases.
vrotterigzl

vrotterigzl

Beginner2022-06-08Added 3 answers

Your points look correct, though your third requires a little thought to justify.
A simpler point is that you know from the median that there are no more than five numbers strictly below 9 and no more than five numbers strictly above 9. It would be helpful if the five smallest could be as small as possible within the constraints so as to make the biggest in the whole set as large as possible. You also want four of the five largest to be as small as possible. So, subject to the constraints, consider
if five of the five smallest are 8s then you could have
8,8,8,8,8,9,9,9,9,10,24
if four of the five smallest are 8s then you could have
1,8,8,8,8,9,9,9,10,10,30
if three of the five smallest are 8s then you could have
1,1,8,8,8,9,9,10,10,11,35
if two of the five smallest are 8s then you could have
1,2,3,8,8,9,10,11,12,13,33
and choose the pattern with the largest maximum.

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