Dan's Mean Numbers

Dan wrote down a list of 10 positive integers. The mean of the (one) smallest integer is 2. The mean of the two smallest integers is 4. The mean of the three smallest integers is 6. The mean of the four smallest integers is 8, so on and so forth. The mean of the ten smallest integers is 20. What is the largest integer?

Details and assumptions

The mean of a set of numbers is the average of all of the numbers in the set.


The answer is 38.

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6 solutions

Ajay Maity
Dec 1, 2013

The mean of the smallest integer is the integer itself. So we know that the smallest number is 2 .

The mean of two smallest integers is 4. We know the smallest integer is 2, so let's assume the 2nd smallest integer to be x. We have 2 + x 2 \frac{2+x}{2} = 4. Solving, we get the second smallest integer as 6 .

The mean of the three smallest integers is 6. We know the two smallest integers are 2 and 6, so let's assume the 3rd smallest integer to be x. We have 2 + 6 + x 3 \frac{2+6+x}{3} = 6. Solving, we get the third smallest integer as 10 .

The mean of the four smallest integers is 8. We know the three smallest integers are 2, 6 and 10, so let's assume the 4th smallest integer to be x. We have 2 + 6 + 10 + x 4 \frac{2+6+10+x}{4} = 8. Solving, we get the fourth smallest integer as 14 .

From these, we observe that the sequence of integers are 2, 6, 10, 14 and so on. So the 10th integer in this case would be 2 + (10 - 1)/2 = 38 \boxed{38} .

Other way to solve is to directly apply the property.

If the sequence a, b, c, d, e, .... has means of 1st smallest integer, 2nd smallest integers, ... forms a sequence (A.P.) with difference D (in our case, 2), then the actual sequence would also be an A.P. with difference 2D (in our case, 4) and the first number in the sequence would be the value corresponding to mean of 1st smallest integer.

Here's a followup question: Do you need to find what all of the first 9 numbers were?

Note: \boxed only applies to the first character after it. To boxed up an expression, you need to type it as \boxed{38} (note the curly brackets), which gives 38 \boxed{38} .

Calvin Lin Staff - 7 years, 6 months ago

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If you get the 1st 3 or 4 numbers, you get an idea of how the sequence is progressing. Or You can use the property I mentioned at the end.

Ajay Maity - 7 years, 6 months ago

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My point is that you don't need to do any of that work in order to find out the last number. There is a lot of excess information given in this problem

Can you solve this problem -
Dan wrote down a list of 10 positive integers. The mean of the nine smallest integers is 18 and the mean of the ten smallest integers is 20. What is the largest integer?

Calvin Lin Staff - 7 years, 6 months ago

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@Calvin Lin Mean of 9 smallest number is 18, so the sum of those 9 numbers would be 18 x 9 = 162. Given, mean of 10 numbers is 20, so sum of those 10 numbers would be 20 x 10 = 200. Last number, i.e. Largest integer = Sum of 10 smallest integers - sum of 9 smallest integers = 200 - 162 = 38.

Ajay Maity - 7 years, 6 months ago

@Calvin Lin I still didn't get how one can find the solution without doing all that work. In your case, last two consecutive sums are given and the last integer is asked. In the problem, if the 5th integer was asked, then nothing of that sort of calculations would be needed. If you could shed some light and tell me what better solution you are talking about.

Ajay Maity - 7 years, 6 months ago
Dan Jodan
Dec 2, 2013

the smallest integer is 2.
because mean increase by 2 so the next integer afrer 2 is plus 4
2 , 6 , 10 , 14 , 18 , 22 , 26 , 30 , 34 , 38 so the largest integer is 38

good

sonu sekar - 7 years, 6 months ago

this is a complicated to understand for average students but a very brilliant solve it is . its a good solution.

akshay chatterjee - 7 years, 6 months ago

Can you explain what you mean by "so the next integer after 2 is plus 4"? Why does that also apply to every case after that?

Calvin Lin Staff - 7 years, 6 months ago

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thats can apply to every case after that because mean increase by 2 for two , three , four .. ....smalest integer . mean 1 smallest integer = 2 ..... sum = 2 mean 2 smallest integer = 4 ..... sum = 8 = 2 + 6 , 6 = 2 + 4 mean 3 smallest integer = 6 ..... sum = 18 = 2 + 6 + 10 , 10 = 6 + 4 .... ... so on

Dan Jodan - 7 years, 6 months ago
Michael Thornton
Dec 20, 2013

The mean of the smallest number is 2. Therefore the smallest number is two.

The mean of the two smallest numbers is 4. The mean is a calculation of the middle value. We know there is a difference of two between 2 and 4, and in order for four to be the middle value there must be a difference of two between 4 and the next number. Therefore the next number is 6.

The mean of the three smallest numbers is six. the mean average is calculated by dividing the sum of all the terms by the amount of terms. We know the mean is six, and there are three terms, the first two of which sum to 8. This gives us the statement:

6 = 2 + 6 + x 3 6 = \frac{2 + 6 + x}{3}

Which can be rearranged to make:

18 = 8 + x 18 = 8 + x

Consequently, the third term must be 10.

Notice that the mean average rises by 2 as we add each term - this means we can use the above method repeatedly to calculate the largest term, but this is an inefficient method. Upon examining our sequence, one might note that it starts at two and increases by four each term - giving us the nth term as:

4 n 2 4n - 2

Using this we could easily find the tenth term by doing:

( 4 × 10 ) 2 = 38 (4 \times 10) - 2 = \boxed{38}

Although I was a bit lazy and just added 4 to two ten times :P

Himanshu Garg
Dec 19, 2013

The mean of one smallest integer is 2, i.e. it is the smallest integer itself because there exists nothing called 'mean of a single no.'. So, first integer is 2.

Now, mean of first and second integer, i.e. (2+a)/2 = 4 & on solving we obtain second integer (a) as 6. Similarly, (2+6+b)/3 = 8, i.e. on solving b = 10.

Now, if we look at first three integers carefully we can clearly identify a series with a common difference of 4. So, we can list down our 10-integers as: 2, 6, 10, 14, 18, 22, 26, 30, 34, 38.

38 is the answer.

The given problem is in Arithmetic Progression -->so first number(a)=2:
-->common difference(d)=4: -->total numbers{n)=10: --> last term=a+(n-1)d==38

Amit Kumar
Dec 6, 2013

As first number is 2 sum of first and 2nd should be 8(4 times 2) so 2nd number is 6. sum of 2nd and 3rd should be 18( 6 times 3) so 3rd number is 10 and so on we get an A.P. with common difference 4
2 6 10 14 18 22 26 30 34 38

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