To log on to a certain computer account, the user must type in a 4-letter password. In such a password, no letter may be repeated, and only the lower case of a letter may be used. How many such 4-letter passwords are possible? (There are 26 letters in the alphabet.)
+36916 26 choices for first letter
25 choices for second letter
24 choices for third letter
23 choices for last letter
26 x 25 x 24 x 23 = ........... choices
