How many four-letter passwords can be formed from the letters {A, B, J, K, X, Z} under the following circumstances. a. If a letter can be repeated? b. If a letter cannot be repeated? 3. Six couples attend a play and are seated in a row of 12 chairs. How many different arrangements are possible if couples are seated together? 4. In how many different arrangements can 3 men and 2 women be seated in a row if no one sits next to a member of the same gender?