AAPL wrote:GMAT Prep

A company plans to assign identification numbers to its employees. Each number is to consist of four different digits from 0 to 9, inclusive, except that the first digit cannot be 0. How many different identification numbers are possible?

A. 3,024
B. 4,536
C. 5,040
D. 9,000
E. 10,000

OA B.

Take the task of creating the identification numbers and break it into stages.

Stage 1: Select the first digit of the identification number
The first digit can be 1, 2, 3, 4, 5, 6, 7, 8, or 9
So, we can complete stage 1 in 9 ways

Stage 2: Select the second digit of the identification number
The second digit can be any digit from 0 to 9 OTHER THAN the digit chosen in stage 1
So, we can complete stage 2 in 9 ways .

Stage 3: Select the third digit of the identification number
The third digit can be any digit from 0 to 9 OTHER THAN the 2 digits digit chosen in stages 1 and 2
So we can complete this stage in 8 ways.

Stage 4: Select the fourth digit of the identification number
The third digit can be any digit from 0 to 9 OTHER THAN the 3 digits digit chosen earlier
So we can complete this stage in 7 ways.

By the Fundamental Counting Principle (FCP), we can complete all 4 stages (and thus create all of the identification numbers) in (9)(9)(8)(7) ways (= 4,536 ways)

Answer: B
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You can also watch a demonstration of the FCP in action: https://www.gmatprepnow.com/module/gmat ... /video/776

Then you can try solving the following questions:

EASY
- https://www.beatthegmat.com/what-should ... 67256.html
- https://www.beatthegmat.com/counting-pr ... 44302.html
- https://www.beatthegmat.com/picking-a-5 ... 73110.html
- https://www.beatthegmat.com/permutation ... 57412.html
- https://www.beatthegmat.com/simple-one-t270061.html


MEDIUM
- https://www.beatthegmat.com/combinatori ... 73194.html
- https://www.beatthegmat.com/arabian-hor ... 50703.html
- https://www.beatthegmat.com/sub-sets-pr ... 73337.html
- https://www.beatthegmat.com/combinatori ... 73180.html
- https://www.beatthegmat.com/digits-numbers-t270127.html
- https://www.beatthegmat.com/doubt-on-se ... 71047.html
- https://www.beatthegmat.com/combinatori ... 67079.html


DIFFICULT
- https://www.beatthegmat.com/wonderful-p ... 71001.html
- https://www.beatthegmat.com/permutation ... 73915.html
- https://www.beatthegmat.com/permutation-t122873.html
- https://www.beatthegmat.com/no-two-ladi ... 75661.html
- https://www.beatthegmat.com/combinations-t123249.html


Cheers,
Brent

AAPL wrote:GMAT Prep

A company plans to assign identification numbers to its employees. Each number is to consist of four different digits from 0 to 9, inclusive, except that the first digit cannot be 0. How many different identification numbers are possible?

A. 3,024
B. 4,536
C. 5,040
D. 9,000
E. 10,000

There are 9 choices for the first digit (1 through 9, inclusive). The second digit can be any of the 10 digits (0 through 9, inclusive) EXCEPT it can't repeat the first digit; thus, there are 9 options for the second digit. The third digit can't repeat either of the first two digits, so there are 8 options. Similarly, the fourth digit can't repeat any of the first 3 digits, so there are 7 options. Thus, the total number of options is 9 x 9 x 8 x 7 = 4,536.

Answer: B