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Credit Card

by zagcollins » Sun Jul 27, 2008 9:30 pm
A credit card number has 6 digits (between 1 to 9). The first two digits are 12 in that order, the third digit is bigger than 6, the fourth is divisible by 3 and the fifth digit is 3 times the sixth. How many different credit card numbers exist?

a)27
b)36
c)72
d)112
e)422
Last edited by zagcollins on Sun Jul 27, 2008 11:22 pm, edited 1 time in total.
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Re: Credit Card

by Neo2000 » Sun Jul 27, 2008 9:56 pm
zagcollins wrote:A credit card number has 5 digits (between 1 to 9). The first two digits are 12 in that order, the third digit is bigger than 6, the fourth is divisible by 3 and the fifth digit is 3 times the sixth. How many different credit card numbers exist?
I'm guessing the first Bold part is a typo??

[spoiler]
Basically, you have to figure out the last 4 digits.
The 3rd digit can be any of 7, 8 or 9. so it can be filled in 3 ways
The 4th can be any of 3,6,9 so it can also be filled in 3 ways
The 5th is depended on the 6th. For the condition to satisfy, you have only 3 pairs (1,3) (2,6) and (3,9)
Therefore total number of ways of filling the last 4digits = 3x3x3 = 27[/spoiler]
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by umaa » Sun Jul 27, 2008 11:18 pm
5 digits - fifth digit is 3 times the sixth;

I don't understand how this 6th digit came.
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