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]

5 digits - fifth digit is 3 times the sixth;

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