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ID number

by Akansha » Sun May 29, 2011 10:25 am
A company assigns ID number to employees. Each is to consist of 4
different digits from 0 to 9 inclusive except 1st digit cannot be 0. How many
different combinations are there?
a. 3024
b. 4536
c. 5040
d. 9000
e. 10000

OA is B
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by sameerballani » Sun May 29, 2011 10:37 am
Akansha wrote:A company assigns ID number to employees. Each is to consist of 4
different digits from 0 to 9 inclusive except 1st digit cannot be 0. How many
different combinations are there?
a. 3024
b. 4536
c. 5040
d. 9000
e. 10000

OA is B
IMO B
Let the Four digit be denoted as ABCD
For first digit A i can choose any digit other than 0 = 9 digits
For B, i can choose any digit other than the one used for A = 9 digits(We can use 0 here)
For C, i can choose any digit other than the one used for A,B = 8 Digits
For D, i can choose any digit other than the one used for A,B,C = 7 Digits

So total combinations possible = 9*9*8*7 = 4536

Thanks
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by edvhou812 » Sun May 29, 2011 9:40 pm
I concur with sameerballani.

4 digit combo with 10 possible digits. Math would normally be 10*9*8*7. However since the first digit can't be zero, the first digit has nine possibilities, and since the second digit may include zero, the second digit will have nine possibilities. So the math will be 9*9*8*7=4,536.
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by itsmebharat » Sun May 29, 2011 11:51 pm
IMO B,

the ques is asking for different combinations, which you might missed while solving.
I am not an Expert, please feel free to suggest if there is an error.
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by smackmartine » Mon May 30, 2011 12:17 am
quick note on tedious multiplication in problems like this.

when you are down to 9 * 9 * 8 * 7 , try to calculate the unit digit of the product.

eg. 9*9 ---> unit digit is always 1, so deciding factor is the unit digit of the product 8*7 i.e 6

Only answer with unit digit as 6 is option B ..choose B and save precious 30 seconds for RC ;)
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by sameerballani » Mon May 30, 2011 3:03 am
good point !!
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