Question

After two numbers are removed from the list $$9,~13,~15,~17,~19,~23,~31,~49,$$ the average and the median each increase by $2$. What is the product of the two numbers that were removed?

Answer 1

247

Explanation:

The average of the original numbers is 176/8 = 22. The median of the original numbers is the average of the middle two numbers: 17 + 19/2 = 18.

Thus, after removing two numbers, we should obtain a list of six numbers whose average is 24 and whose median is 20.

For the median of six numbers to be 20, the middle two numbers in that list must add up to 40. Searching our original list for pairs of numbers that add up to 40, we find two such pairs: 9, 31 and 17, 23. But 9 and 32 can't be the middle numbers after we remove two numbers, so 17 and 23 must be the middle numbers. This tells us that one of the removed numbers must be 19.

For the average of six numbers to be 24, the sum of the six numbers must be 6 ∙ 24 = 144. This is 32 less than the original sum of 176, so if one of the removed numbers is 19, the other must be 32 - 19 = 13.

Therefore, the product of the two removed numbers is 13 ∙ 19 = 247.

Answer 2

The removed numbers are 13 and 19, and the product is:

13*19 = 247

Explanation:

We have the set:

{9, 13, 15, 17, 19, 23, 31, 49}

The original median is the number that is just in the middle of the set (in a set of 8 numbers, we take the average between the fourth and fifth numbers)

then the median is:

(17 + 19)/2 = 18

and the mean is:

(9 + 13 + 15 + 17 + 19 + 23 + 31 + 49)/8 = 22

We want to remove two numbers such that the mean and the median increase by two.

Is immediate to notice that if we want the median to increase by two, we need to remove the number 19 and one number smaller than 17.

Then the median will be equal to:

(17 + 23)/2 = 20

which is 2 more than the previous median.

because 19 assume that we remove the 19 and number N.

To find the value of N, we can solve for the new mean:

((9 + 13 + 15 + 17 + 23 + 31 + 49 - N)/6 = 22 + 2

(this means that if we remove the number 19 and the number N, the mean increases by 2.

(9 + 13 + 15 + 17 + 23 + 31 + 49 - N)/6 = 22 + 2

(9 + 13 + 15 + 17 + 23 + 31 + 49 - N) = 24*6 = 144

157 - N = 144

157 - 144 = N = 13

This means that the other number we need to remove is 13

Then we remove the numbers 13 and 19

The product of the two removed numbers is:

13*19 =247