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Answer:
a) Number of passwords: 2763633600
b) Number of passwords: 2487270240
Step-by-step explanation:
We have:
10 digits,
26 letters, and
4 special characters.
We have to compute the number of passwords that satisfy the following conditions:
a) Strings of length 6. Characters can be special characters, digits, or letters, with no repeated characters.
We can use the three types of characters, as long as they do not repeat.
We have a total of 10+26+4=40 characters.
This means we have 40 options for the first place, (40 - 1) for the second place (as the first place already use one of the options available), and so on up to 6 places.
We can write this as:
b. Strings of length 6. Characters can be special characters, digits, or letters, with no repeated characters. The first character can not be a special character.
In this case we add a restriction: the first character can not be a special character.
Then, for the first place we have only 40-4=36 options (digits and letters).
For the second place we can use the 39 options available (digits, letters and special characters).
Then we have: