Math
posted by Anonymous on .
A password consists of 2 letters and 2 digits (09). The letters and digits cannot be repeated. How many passwords are possible?

C=27!/25! * 10!/8! * 4!/2!2!
The last term represents the number of permuations of two letters, two numbers can be done (six ways). 
my approach:
first lets choose 2 letters and 2 numbers,
that is C(26,2)*C(10,2)
= 325*45
= 14625
let's consider one of these, say, DK45
now that could be arranged in 4! ways
so the total number of passwords is
14625*4! = 351000
bobpursley, when did they introduce that 27th letter? 
A student can create a 3 digit computer password from the numbers 0 to 9. If the digits cannot be repeated, what is the probability of someone guessing the correct password?