Pin
Probability
Level
1
My pin# is 4 digits long, it's an even number and all of the digits (0-9 are the options) are different. Even knowing all of this, what chance do you have of guessing it in a single guess?
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1 solution
Since the pin# is even, the last digit must be 0, 2, 4, 6, or 8 -- 5 options.
Since all of the digits are distinct, that leaves 9 options for the 3rd digit, then 8 for the 2nd digit, and then 7 for the 1st digit. 5 × 9 × 8 × 7 = 2 5 2 0 . So, the chance of guessing her pin correctly in one guess is still only 2 5 2 0 1 .
That said, if she hadn't said anything about it, it would have been 1 0 4 1 = 1 0 0 0 0 1 so it's a much weaker pin now. So don't tell people things about your pin/passwords!