In how many W ways can a four-letter password be chosen, using the letters A, B, C, D, E, and/or F, such that at least one letter is repeated within the password?	Choose the correct option.
A) 720  B) 864  C) 900  D) 936  E) 1,296
Total number of four letter passwords = 6*6*6*6= 
<gadget id="calculator">6*6*6*6</gadget>
<output>1_296</output>
1296 ------(1)   Total number of passwords in which no letter repeats= 6C4*4!= 15*24= 
<gadget id="calculator">15*24</gadget>
<output>360</output>
360------(2)   therefore required value W= (1)-(2)= 1296-360=936.936
<result>D</result>
In how many W ways can a four-letter password be chosen, using the letters A, B, C, D, E, and/or F, such that at least one letter is repeated within the password?