Jessica is choosing a 3 letter password from the letters A, B, C, and D. The password cannot have the same letter repeated in it. How many such passwords are possible?

There are 6 passwords for each letter it starts with so a total of 24:

ABC, ABD, ACB, ACD, ADB, ADC, BAC, BAD, BCA, BCD, BDA, BDC,

CAB, CAD, CBA, CBD, CDA, CDB, DAB, DAC, DBA, DBC, DCA, DCB

3 slots

4 letters

4 choices for the first slot

3 choices for 2nd (since we used one for the first slot)

2 choices for the 3rd slot (since we used another for 2nd slot)

so total number of passwords is 4*3*2=24 passwords