In how many H 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 answer:
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 H= (1)-(2)= 1296-360=936.936
<result>D</result>
In how many H 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?