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Manager  Status: Trying to survive
Joined: 29 Jun 2011
Posts: 153
GMAt Status: Quant section
Concentration: Finance, Real Estate
Schools: WBS (D)
GMAT Date: 12-30-2011
GPA: 3.2
How many 3 digit integers can be chosen such that none of  [#permalink]

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20 00:00

Difficulty:   75% (hard)

Question Stats: 52% (02:01) correct 48% (02:04) wrong based on 273 sessions

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How many 3 digit integers can be chosen such that none of the digits appear more than twice and none of the digits equal zero ?

A. 729
B. 720
C. 648
D. 640
E. 576

what is the wrong of my method which is :
3*(9*9*8)
where first digit can be chosen by 9 ways
second digit can be chosen by 9 ways
third digit can be chosen by 8 ways
and since it's anagram of ( SSD ) it can be arranged in 3 ways so the answer IMO is 3*(9*9*8)= 1944 which is wrong

_________________
How can i lose my faith in life's fairness when i know that the dreams of those who sleep on the feathers are not more beautiful than the dreams of those who sleep on the ground? - Jubran Khaleel Jubran

Originally posted by Silver89 on 27 Jan 2012, 07:42.
Last edited by Silver89 on 27 Jan 2012, 08:35, edited 1 time in total.
Manager  Status: Trying to survive
Joined: 29 Jun 2011
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GMAt Status: Quant section
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Schools: WBS (D)
GMAT Date: 12-30-2011
GPA: 3.2
Re: How many 3 digit integers can be chosen such that none of  [#permalink]

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1
1
Thanks,
In fact you can solve it in another way too which is
number of ways without restriction = 9*9*9 = 729
number of ways that break restriction = 9*1*9 = 9
so the answer is 729-9 = 720

My question why can't i say
first digit can be chosen by 9 ways
second digit can be chosen by 9 ways
third digit can be chosen by 8 ways
and we can arrange it in 3 ways, thus the answer is 9*9*8*3
_________________
How can i lose my faith in life's fairness when i know that the dreams of those who sleep on the feathers are not more beautiful than the dreams of those who sleep on the ground? - Jubran Khaleel Jubran
Math Expert V
Joined: 02 Sep 2009
Posts: 59087
How many 3 digit integers can be chosen such that none of  [#permalink]

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3
1
Silver89 wrote:
How many 3 digit integers can be chosen such that none of the digits appear more than twice and none of the digits equal zero ?

A. 729
B. 720
C. 648
D. 640
E. 576

Thanks,
In fact you can solve it in another way too which is
number of ways without restriction = 9*9*9 = 729
number of ways that break restriction = 9*1*9 = 9
so the answer is 729-9 = 720

My question why can't i say
first digit can be chosen by 9 ways
second digit can be chosen by 9 ways
third digit can be chosen by 8 ways
and we can arrange it in 3 ways, thus the answer is 9*9*8*3

Yes, the easiest way would be as you pointed out:
A. Numbers without restriction = 9^3;
B. 3-digit integers with all alike digits = 9 (111, 222, ..., 999);

A-B=9^3-9=720.

Now, if we go your way it should be 9*8*7+9*8*3=720, where 9*8*7 is # of 3-digit integers with all distinct digits and 9*8*3 is # of 3-digit integers with two alike digits (XXY - 9*8 and multiplied by 3!/2! to get different permutations).

So, as there are two cases you cannot combine them in one formula and apply one factorial correction (3!/2!) to it.

Hope it's clear.
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Posts: 8
Concentration: Marketing, General Management
Re: How many 3 digit integers can be chosen such that none of  [#permalink]

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1
Bunuel wrote:
Silver89 wrote:
Thanks,
In fact you can solve it in another way too which is
number of ways without restriction = 9*9*9 = 729
number of ways that break restriction = 9*1*9 = 9
so the answer is 729-9 = 720

My question why can't i say
first digit can be chosen by 9 ways
second digit can be chosen by 9 ways
third digit can be chosen by 8 ways
and we can arrange it in 3 ways, thus the answer is 9*9*8*3

Yes, the easiest way would be as you pointed out:
A. Numbers without restriction = 9^3;
B. 3-digit integers with all alike digits = 9 (111, 222, ..., 999);

A-B=9^3-9=720.

Now, if we go your way it should be 9*8*7+9*8*3=720, where 9*8*7 is # of 3-digit integers with all distinct digits and 9*8*3 is # of 3-digit integers with two alike digits (XXY - 9*8 and multiplied by 3!/2! to get different permutations).

So, as there are two cases you cannot combine them in one formula and apply one factorial correction (3!/2!) to it.

Hope it's clear.

Sorry I don't understand both ways you mentioned. I dont know what is wrong with my way
9 ways for the first digit
9 ways for the second digit
8 ways for the last one
=> 9 x 9 x 8 = 648 ways. please explain! Many thanks
Math Expert V
Joined: 02 Sep 2009
Posts: 59087
Re: How many 3 digit integers can be chosen such that none of  [#permalink]

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1
1
HuongShashi wrote:
Bunuel wrote:
Silver89 wrote:
Thanks,
In fact you can solve it in another way too which is
number of ways without restriction = 9*9*9 = 729
number of ways that break restriction = 9*1*9 = 9
so the answer is 729-9 = 720

My question why can't i say
first digit can be chosen by 9 ways
second digit can be chosen by 9 ways
third digit can be chosen by 8 ways
and we can arrange it in 3 ways, thus the answer is 9*9*8*3

Yes, the easiest way would be as you pointed out:
A. Numbers without restriction = 9^3;
B. 3-digit integers with all alike digits = 9 (111, 222, ..., 999);

A-B=9^3-9=720.

Now, if we go your way it should be 9*8*7+9*8*3=720, where 9*8*7 is # of 3-digit integers with all distinct digits and 9*8*3 is # of 3-digit integers with two alike digits (XXY - 9*8 and multiplied by 3!/2! to get different permutations).

So, as there are two cases you cannot combine them in one formula and apply one factorial correction (3!/2!) to it.

Hope it's clear.

Sorry I don't understand both ways you mentioned. I dont know what is wrong with my way
9 ways for the first digit
9 ways for the second digit
8 ways for the last one
=> 9 x 9 x 8 = 648 ways. please explain! Many thanks

We want to find the number of 3-digit integers, without 0, in which none of the digits appear more than twice. So, the number should be XXY (2 digits are alike) or XYZ (all digits are distinct).

Now, {all 3-digit integers, without 0} = {3-digit integers with all the same digits} + {3-digit integers where 2 digits are alike} + {3-digit integers with distinct digits}.

Therefore,
{3-digit integers where 2 digits are alike} + {3-digit integers with distinct digits} = {all 3-digit integers, without 0} - {3-digit integers with the same digits}.

{3-digit integers with 2 digits are alike} + {3-digit integers with distinct digits} = 9^3 - 9.

Hope it helps.
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Posts: 61
WE: General Management (Non-Profit and Government)
Re: How many 3 digit integers can be chosen such that none of  [#permalink]

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HuongShashi wrote:
Bunuel wrote:
Silver89 wrote:
Thanks,
In fact you can solve it in another way too which is
number of ways without restriction = 9*9*9 = 729
number of ways that break restriction = 9*1*9 = 9
so the answer is 729-9 = 720

My question why can't i say
first digit can be chosen by 9 ways
second digit can be chosen by 9 ways
third digit can be chosen by 8 ways
and we can arrange it in 3 ways, thus the answer is 9*9*8*3

Yes, the easiest way would be as you pointed out:
A. Numbers without restriction = 9^3;
B. 3-digit integers with all alike digits = 9 (111, 222, ..., 999);

A-B=9^3-9=720.

Now, if we go your way it should be 9*8*7+9*8*3=720, where 9*8*7 is # of 3-digit integers with all distinct digits and 9*8*3 is # of 3-digit integers with two alike digits (XXY - 9*8 and multiplied by 3!/2! to get different permutations).

So, as there are two cases you cannot combine them in one formula and apply one factorial correction (3!/2!) to it.

Hope it's clear.

Sorry I don't understand both ways you mentioned. I dont know what is wrong with my way
9 ways for the first digit
9 ways for the second digit
8 ways for the last one
=> 9 x 9 x 8 = 648 ways. please explain! Many thanks

As bunnel aptly said, it is quite simple to follow two steps :

1) 9*9*9 =729 ;this includes the no , which are repeated more than twice.
2) we have 9 no's excluding 0 ,whose all digits are same.

Hence , total no =729-9=720

Manager  Joined: 10 Mar 2013
Posts: 167
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Direct with restriction:
Let m = number of three-digit integers with one digit appearing twice
m=9*1*8=72
However, there are C(3,2) ways that this condition can occur.
M=Total Appearing Twice=m*C(3,2)=72*3=216

Let z=number of three-digit integers with each digit appearing once.
z=9*8*7=504
There are C(3,3) ways that this condition can occur
Z=Total Appearing Once=z*C(3,3)=504*1=504

Total satisfying the problem condition=M+Z=216+504=720

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Posts: 1871
Re: How many 3 digit integers can be chosen such that none of  [#permalink]

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Silver89 wrote:
How many 3 digit integers can be chosen such that none of the digits appear more than twice and none of the digits equal zero ?

A. 729
B. 720
C. 648
D. 640
E. 576

what is the wrong of my method which is :
3*(9*9*8)
where first digit can be chosen by 9 ways
second digit can be chosen by 9 ways
third digit can be chosen by 8 ways
and since it's anagram of ( SSD ) it can be arranged in 3 ways so the answer IMO is 3*(9*9*8)= 1944 which is wrong

The simplest way to solve this is to find the numbers without any conditions applied and then from that subtract the numbers the satisfy the conditions.

Total 3-digit numbers without any digit being 0 = 9 * 9 * 9 = 729.
Out of these only 111, 222, 333, ... , 999 violate the given condition that a digit cannot be repeated more than twice.
Hence there are 9 such numbers.
Hence 729 - 9 = 720.

Hence option (B).

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Intern  Joined: 08 Sep 2014
Posts: 3
Re: How many 3 digit integers can be chosen such that none of  [#permalink]

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Let's say this number is ABC.

Let's calculate how many numbers with two similar non zero digits and how many numbers with distinct non zero digits:

1. Two similar none zero digits: A has 9 possibilities, B has 1 possibility and C has 8 possibilities. So 9.1*8=72 different numbers. However, this configuration can be modeled in 3 ways, depending on the position of the similar numbers: XXY, XYX, YXX, therefore 72*3=216.

2. Distinct numbers: A has 9 possibilities, B has 8 possibilities and C has 7 possibilities, therfore: 9*8*7=504.

In total we have 504+216=720.
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How many 3 digit integers can be chosen such that none of  [#permalink]

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How many 3 digit integers can be chosen such that none of the digits appear more than twice and none of the digits equal zero?

9/9*9/10*9/10=729/900
729-9 triples=720
B. 720
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Joined: 30 Apr 2013
Posts: 75
Re: How many 3 digit integers can be chosen such that none of  [#permalink]

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I solved it by 9 * 9 * 8

First digit nine ways
Second digit nine ways since we can repeat the number twice
Third number 8

Why is it wrong?
Math Expert V
Joined: 02 Sep 2009
Posts: 59087
Re: How many 3 digit integers can be chosen such that none of  [#permalink]

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santro789 wrote:
I solved it by 9 * 9 * 8

First digit nine ways
Second digit nine ways since we can repeat the number twice
Third number 8

Why is it wrong?

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Joined: 27 Nov 2006
Posts: 55
Re: How many 3 digit integers can be chosen such that none of  [#permalink]

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TooLong150 wrote:
Direct with restriction:
Let m = number of three-digit integers with one digit appearing twice
m=9*1*8=72
However, there are C(3,2) ways that this condition can occur.
M=Total Appearing Twice=m*C(3,2)=72*3=216

Let z=number of three-digit integers with each digit appearing once.
z=9*8*7=504
There are C(3,3) ways that this condition can occur
Z=Total Appearing Once=z*C(3,3)=504*1=504

Total satisfying the problem condition=M+Z=216+504=720

B

Can someone explain the logic behind "There are C(3,3) ways that this condition can occur". I've never seen this.
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How many 3 digit integers can be chosen such that none of  [#permalink]

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Silver89 wrote:
How many 3 digit integers can be chosen such that none of the digits appear more than twice and none of the digits equal zero ?

A. 729
B. 720
C. 648
D. 640
E. 576

SOLUTION 1 (CASES)
$$digits={1,2,3,4,5,6,7,8,9}=9$$
$$[000:all.different]…9•8•7=72•7=490+14=504$$
$$[110:pair.different]…9•1•8•arrangements(3!/2!)=72•3=216$$
$$total=504+216=720$$

SOLUTION 2 (TOTAL-NOTCASE)
$$digits={1,2,3,4,5,6,7,8,9}=9$$
$$[XXX:any.digit]…9•9•9=729$$
$$[333:triple]…9•1•1=9$$
$$total-not=729-9=720$$ How many 3 digit integers can be chosen such that none of   [#permalink] 23 Sep 2019, 08:00
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