Passwords Are Made Using 4 Letters And 2 Digits. How Many Passwords Can Be Made If Repetition Of Letters And Digits Is Allowed?

For each letter or digit you choose, one less choice is available for the next.

26*25*24*23*10*9*(rearrangements) = # of passwords

There are 25 rearrangements, so 807,300,000 passwords can be formed.
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Consider _x_x_x_x_, where each x is a letter and each _ is a place where a number may be put. We can put one or two numbers in each _, so there are 5 choices for each number. 5*5=25 rearrangements.