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# The security gate at a storage facility requires a fivE

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Manager
Joined: 26 Sep 2010
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The security gate at a storage facility requires a fivE [#permalink]

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23 Feb 2011, 22:05
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35% (medium)

Question Stats:

64% (02:05) correct 36% (01:25) wrong based on 309 sessions

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The security gate at a storage facility requires a five-digit lock code. If the lock code must consist only of digits from 1 through 7, inclusive, with no repeated digits, and the first and last digits of the code must be odd, how many lock codes are possible?

A. 120
B. 240
C. 360
D. 720
E. 1440
[Reveal] Spoiler: OA

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24 Feb 2011, 04:16
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IndigoIntentions wrote:
The security gate at a storage facility requires a five-digit lock code. If the lock code
must consist only of digits from 1 through 7, inclusive, with no repeated digits, and
the first and last digits of the code must be odd, how many lock codes are possible?

a)120
b)240
c)360
d)720
e)1440

X-X-X-X-X --> there are 4 odd digits from 1 to 7, inclusive thus options for the first and the last X's are: 4-X-X-X-3. Other X's can take following values: 4-5-4-3-3 --> 4*5*4*3*3=720.

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24 Feb 2011, 04:21

Digit #1: has to be odd => 4 possibilities (1,3,5,7)

Digit #5: has to be odd => 3 possiblities left (one possibility has already been used for digit #1)

Digit #2: any of the 7 possible digits (1,2,...7) less the 2 that have already been used for Digits #1 & #5 => 5 possibilities

Digit #3: any of the 7 possible digits (1,2,...7) less the 3 that have already been used for Digits #1, #2 & #5 => 4 possibilities

Digit #4: any of the 7 possible digits (1,2,...7) less the 4 that have already been used for Digits #1, #2, #3 & #5 => 3 possiblities

Thus, the number of possiblities is 4*3*5*4*3 = 720
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24 Feb 2011, 04:57
1
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4 * 3 * 5!/2!

= 12 * (5 * 4 * 3* 2!)/2!

= 12 * 60 = 720

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18 Apr 2013, 22:17
Bunuel wrote:
IndigoIntentions wrote:
The security gate at a storage facility requires a five-digit lock code. If the lock code
must consist only of digits from 1 through 7, inclusive, with no repeated digits, and
the first and last digits of the code must be odd, how many lock codes are possible?

a)120
b)240
c)360
d)720
e)1440

X-X-X-X-X --> there are 4 odd digits from 1 to 7, inclusive thus options for the first and the last X's are: 4-X-X-X-3. Other X's can take following values: 4-5-4-3-3 --> 4*5*4*3*3=720.

Hi bunuel,

I have a small doubt , why not we are considering that the first and last digit can occur in 2 ways and the middle 3 digits can occur in 6 ways.

for example middle 3 digits can be arranged in 3! ways among themselves after selecting 5c1 4c1 3c1 .

My question may be stupid, please correct my doubt.

Thanks
Math Expert
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19 Apr 2013, 03:28
Expert's post
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tarunjagtap wrote:
Bunuel wrote:
IndigoIntentions wrote:
The security gate at a storage facility requires a five-digit lock code. If the lock code
must consist only of digits from 1 through 7, inclusive, with no repeated digits, and
the first and last digits of the code must be odd, how many lock codes are possible?

a)120
b)240
c)360
d)720
e)1440

X-X-X-X-X --> there are 4 odd digits from 1 to 7, inclusive thus options for the first and the last X's are: 4-X-X-X-3. Other X's can take following values: 4-5-4-3-3 --> 4*5*4*3*3=720.

Hi bunuel,

I have a small doubt , why not we are considering that the first and last digit can occur in 2 ways and the middle 3 digits can occur in 6 ways.

for example middle 3 digits can be arranged in 3! ways among themselves after selecting 5c1 4c1 3c1 .

My question may be stupid, please correct my doubt.

Thanks

Consider this: two digit code XX, each digit must be distinct and can be 1, 2 or 3.

First digit can take 3 values (1, 2, or 3) and the second can take 2 values, total 3*2=6 codes:
12
13
21
23
31
32.

Similarly, for the original question: the first digit can take 4 values and the last digit can take 3 values, total 4*3. The same way for the middle 3 digits: the second digit can take 5 values (7 minus two we already used for first and last), the third 4 and the fourth 3.

Total: (4*3)*5*4*3=720.

Hope it's clear.
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Re: The security gate at a storage facility requires a fivE [#permalink]

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24 Apr 2013, 23:13
IndigoIntentions wrote:
The security gate at a storage facility requires a five-digit lock code. If the lock code must consist only of digits from 1 through 7, inclusive, with no repeated digits, and the first and last digits of the code must be odd, how many lock codes are possible?

A. 120
B. 240
C. 360
D. 720
E. 1440

1. There is one large group i.e., digits 1 through 7, from which 5 digits should be selected.
2. The first and the last digits have a constraint that they should be odd. There is also a general constraint that each digit should be different.
3. Considering the above constraints the first and the last digits can be selected in $$4P2$$ ways. 4 because there are 4 odd digits out of which they can be selected and because there should be no repetition it is $$4P2$$ ways for those 2 positions.
4. The second digit cannot have the same digit as the first and the last digit. Therefore it can be chosen out of 7-2=5 digits. Similarly the third digit cannot have the same digit as the first, second and last. So it can be chosen in 4 ways and the fourth digit similarly in 3 ways.
5. So the number of lock codes possible is $$4P2 * 5* 4*3 = 720$$

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Re: The security gate at a storage facility requires a fivE [#permalink]

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20 Jun 2013, 17:23
We have four odd numbers in this set 1, 3, 5, 7

First digit is 4C1 = 4
Last digit is 3C1 = 3 (we have to exclude the odd digit selected in the previous step.

mid digits are 3 of the remaining 5 digits (2, 4 , 6, in addition to two not selected odd digits) for the 2nd, 3rd and 4th digits in the code

mid digits 5C3 = 60

4*60*3= 720

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Re: The security gate at a storage facility requires a fivE [#permalink]

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25 Aug 2013, 03:56
Odd digits between 1 and 7
1 3 5 7 ==> 4
first and last place being fixed, can be filled with Odd number is
4* 5 *4* 3* 3

Total 720 combinations

Ans: D
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Re: The security gate at a storage facility requires a fivE [#permalink]

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06 May 2015, 13:27
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Re: The security gate at a storage facility requires a fivE [#permalink]

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02 Jul 2016, 16:52
Hello from the GMAT Club BumpBot!

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Re: The security gate at a storage facility requires a fivE   [#permalink] 02 Jul 2016, 16:52
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