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Re: The ten children in a certain group contributed a total of 28 pieces [#permalink]
Let:
x students donate (n-1) clothes.
y students donate n clothes.
z students donate (n+1) clothes.

From the problem statement we can write these two equations:

x+y+z = 10 ........ (I)
(n-1)x + ny + (n+1)z = 28 ...........(II)

Simplify equation (II)

(n-1)x + ny + (n+1)z = 28
nx-x+ny+nz+z = 28
n(x+y+z)+z-x = 28

Put x+y+z = 10 from equation (I)

10n+z-x = 28
or, z = x+28-10n

Put n=1:
z = x+18
This is not possible since z has to be less than or equal to 10 (because there are only 10 students)

Put n=2:
z= x+8
If n=2, then number of students donating 3 clothes will be z.

Put n=3:
z = x-2
If n=3, then the number of students donating 3 clothes will be y.
y = 12-2x .......(Use the equation x+y+z=10)

Put n=4:
x = z+12
This is not possible since x<=10.

So we get 2 expressions for number of students donating 3 clothes = (x+8) and (12-2x)

Notice that:

0 can be obtained by putting x=6 in (12-2x).
9 can be obtained by putting x=1 in (x+8).

5 can never be obtained since x is a non-negative integer. So, the minimum value of (x+8) is 8.
(12-2x) is an even number so it can never be 5.

So, option D is the correct answer.
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Re: The ten children in a certain group contributed a total of 28 pieces [#permalink]
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Arghi wrote:
Thanks you gmatophobia, This is the method I also used to solve this problem. But it is quite time consuming and I was wondering if there is another quicker way to approach this question.


This can be solved in under 3 min if we simply look at the options from down below :

A 0
B 5
C 9

C. Well 9*3 + 1*1 = 27 + 1 = 28 and we have 28 shirts exactly hence 3 shirts contributed by 9 boys possible

B. Well 5*3 = 15, so 28-15 = 13 contributions are left, 5 boys have already contributed, so rest 5 must contribute 13
since range is 2; only 2,4 or 4,5 or 1,2 are possible contributions in this range
Using weighted average :
2a + 4b = 13 not possible ( 2*5 = 10 and 4*5 = 20 weighted average possible but sum must be even and it is not )
4a + 5b = 13 not possible ( 4*5 = 20 and 5*5 = 25 in no way the weighted avg can be below 20 )
1a + 2b = 13 not possible ( 1*5 = 5 and 2*5 = 10 in no way weighted avg can be above 10 )

A. Since contributors with 3 shirts are not present,
Simply by observation we can notice in above case (B) 2*10 = 20 can be contributed by 10 people, now rest 8 we can divide among 4 contributors who contributes 4 shirts each.
2a + 4b = 28 and a + b = 10 can have Integer solutions as a = 6, b = 4

So A, C possible
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The ten children in a certain group contributed a total of 28 pieces [#permalink]
1
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Pre-thinking
Since the range =2, the list can have two formats:
a) 2 _ _ _ _ _ _ _ _ 4
b) 1 _ _ _ _ _ _ _ _ 3

I. To have zero 3's, given that the range = 2, we have a list that begun with 2 and ends with 4 (if it begin with 1, it would necessarily finish with 3, with contradicts the initial assumption of zero 3's). By trying an error, you will quickly combine 2's (5) and 4's (3) that sum up to 28.

III. If you have 9 3's, it sums up to 27 and the last available slot in the list must necessarily be filled with 1 so the sum of the 10 elements can be 28, which makes sense with the range = 2 said in the question.

II. Let's see each arrangement
a) we need to fill five consecultive blanks 3's and three blanks (consecultive or not) with 2's or 4's. After fill with the 3's, the sum left is 28-(5*3+2+4)=7. It is impossible combine an natural amount of 2's and 4's to sum up 7.
b) we need to fill FOUR (because we already have a 3 in the list) consecultive blanks with 3's and four blanks with 2's or 1's.
1 _ _ _ _ 3 3 3 3 3
After fill with the 3's, the sum left is 28-(5*3+1)=12. The maximum that we can fill is 4 2's = 8. So this arrange is also impossible.­
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Re: The ten children in a certain group contributed a total of 28 pieces [#permalink]
Bunuel GMATNinja can you tell a shorter route for this
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Re: The ten children in a certain group contributed a total of 28 pieces [#permalink]
chetan2u , requesting to provide an easier solution ..
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Re: The ten children in a certain group contributed a total of 28 pieces [#permalink]
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Expert Reply
sayan640

If I were to do this question, I would look at the average of all, which would be 28/10 or 2.8
Thus, the average is closer to 3.

Possible ranges: 1-3, 2-4. No other range is possible.

Options:
1) 0
Only range is 2-4. This means few are 2 and remaining are 4 and this should make the average 2.8
Weighted average: 2…….2.8…….4 => 2…..(0.8)….2.8…..(1.2)……4
Number of 2s to total = 10*1.2/(0.8+1.2) = 12/2 = 6. Thus, six of 2s and four of 4s.

2) 5
1-3 range is not possible. Only possibility is 2-4
So total of remaining 5 is 28-(5*3) = 13.
We have to use five of 2s and 4s to get 13, that is 2x+4(5-x)=13. But 2x+4(5-x) will always be even.
Not possible

3) 9
1-3 range is a possibility
The 10th number would be 28-3*9 or 1. Possible.

Thus, 0 and 9 are possible
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Re: The ten children in a certain group contributed a total of 28 pieces [#permalink]
chetan2u,
I faced problem in understanding the highlighted part. 
Which test do you perform to say at the outset that "1-3 range is not possible. " or "1-3 range is  possible." Please help.
chetan2u wrote:
sayan640

If I were to do this question, I would look at the average of all, which would be 28/10 or 2.8
Thus, the average is closer to 3.

Possible ranges: 1-3, 2-4. No other range is possible.

Options:
1) 0
Only range is 2-4. This means few are 2 and remaining are 4 and this should make the average 2.8
Weighted average: 2…….2.8…….4 => 2…..(0.8)….2.8…..(1.2)……4
Number of 2s to total = 10*1.2/(0.8+1.2) = 12/2 = 6. Thus, six of 2s and four of 4s.

2) 5
1-3 range is not possible. Only possibility is 2-4
So total of remaining 5 is 28-(5*3) = 13.
We have to use five of 2s and 4s to get 13, that is 2x+4(5-x)=13. But 2x+4(5-x) will always be even.
Not possible

3) 9
1-3 range is a possibility
The 10th number would be 28-3*9 or 1. Possible.

Thus, 0 and 9 are possible

­
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Re: The ten children in a certain group contributed a total of 28 pieces [#permalink]
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Expert Reply
sayan640 wrote:
chetan2u,
I faced problem in understanding the highlighted part. 
Which test do you perform to say at the outset that "1-3 range is not possible. " or "1-3 range is  possible." Please help.
chetan2u wrote:
sayan640

If I were to do this question, I would look at the average of all, which would be 28/10 or 2.8
Thus, the average is closer to 3.

Possible ranges: 1-3, 2-4. No other range is possible.

Options:
1) 0
Only range is 2-4. This means few are 2 and remaining are 4 and this should make the average 2.8
Weighted average: 2…….2.8…….4 => 2…..(0.8)….2.8…..(1.2)……4
Number of 2s to total = 10*1.2/(0.8+1.2) = 12/2 = 6. Thus, six of 2s and four of 4s.

2) 5
1-3 range is not possible. Only possibility is 2-4
So total of remaining 5 is 28-(5*3) = 13.
We have to use five of 2s and 4s to get 13, that is 2x+4(5-x)=13. But 2x+4(5-x) will always be even.
Not possible

3) 9
1-3 range is a possibility
The 10th number would be 28-3*9 or 1. Possible.

Thus, 0 and 9 are possible

­


First we know that only possibilities are 1-3 and 2-4.

Next, for number of 3s to be NINE.
Can the range be 2-4?
No, as we have only one number to choose, so for range to be 2, the number has to be two away from 3, that is 1-3 or 3-5. Now, 3-5 is not possible as the total then will be more than 28.
Only possibility 1-3.

Next, for number of 3s to be FIVE.
Can the range be 1-3? The possible maximum sum will be 1*1+4*2+5*3 or 24.
Thus, 2-4 is the only possibility

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Re: The ten children in a certain group contributed a total of 28 pieces [#permalink]
Thank you chetan2u
chetan2u wrote:
sayan640 wrote:
chetan2u,
I faced problem in understanding the highlighted part. 
Which test do you perform to say at the outset that "1-3 range is not possible. " or "1-3 range is  possible." Please help.
chetan2u wrote:
sayan640

If I were to do this question, I would look at the average of all, which would be 28/10 or 2.8
Thus, the average is closer to 3.

Possible ranges: 1-3, 2-4. No other range is possible.

Options:
1) 0
Only range is 2-4. This means few are 2 and remaining are 4 and this should make the average 2.8
Weighted average: 2…….2.8…….4 => 2…..(0.8)….2.8…..(1.2)……4
Number of 2s to total = 10*1.2/(0.8+1.2) = 12/2 = 6. Thus, six of 2s and four of 4s.

2) 5
1-3 range is not possible. Only possibility is 2-4
So total of remaining 5 is 28-(5*3) = 13.
We have to use five of 2s and 4s to get 13, that is 2x+4(5-x)=13. But 2x+4(5-x) will always be even.
Not possible

3) 9
1-3 range is a possibility
The 10th number would be 28-3*9 or 1. Possible.

Thus, 0 and 9 are possible

­


First we know that only possibilities are 1-3 and 2-4.

Next, for number of 3s to be NINE.
Can the range be 2-4?
No, as we have only one number to choose, so for range to be 2, the number has to be two away from 3, that is 1-3 or 3-5. Now, 3-5 is not possible as the total then will be more than 28.
Only possibility 1-3.

Next, for number of 3s to be FIVE.
Can the range be 1-3? The possible maximum sum will be 1*1+4*2+5*3 or 24.
Thus, 2-4 is the only possibility

Posted from my mobile device
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Re: The ten children in a certain group contributed a total of 28 pieces [#permalink]
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