A student needs to create a 7-character alpha-numeric passcode (that is, at least one letter and one digit must be included). She uses lower case letters from her name "venessa see" and digits 1 to 9.
- (a)
Find the number of different passcodes if all the letters and numbers are distinct such that
- (i)
they begin with a letter and end with a digit.
- (ii)
all the letters and digits are to be separated.
- (i)
- (b)
Find the number of different passcodes if there are 3 identical letters, and all other letters and digits are distinct.
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Worked solution
6(a)(i)
1. Identify the number of available distinct letters and digits.
Distinct letters in "venessa see": (5 letters)
Distinct digits from 1 to 9: (9 digits)
Total distinct characters available =
2. Determine the number of choices for the restricted first and last positions.
First position (must be a letter): choices
Last position (must be a digit): choices
Number of characters remaining after filling the first and last positions =
3. Determine the number of ways to arrange characters in the remaining positions and calculate the total number of passcodes.
Remaining positions to fill =
Ways to arrange the remaining 12 characters into 5 positions =
Total number of passcodes =
6(a)(ii)
1. Determine the possible patterns to ensure all letters and digits are separated.
For all letters (L) and digits (D) to be separated, they must strictly alternate. Since the passcode is 7 characters long, there are two possible patterns:
L-D-L-D-L-D-L(4 letters, 3 digits)D-L-D-L-D-L-D(3 letters, 4 digits)
2. Calculate the number of ways for the `L-D-L-D-L-D-L` pattern.
There are distinct letters (v, e, n, s, a) and distinct digits.
Choosing and arranging 4 letters and 3 digits:
3. Calculate the number of ways for the `D-L-D-L-D-L-D` pattern and find the total.
Choosing and arranging 3 letters and 4 digits:
Total number of passcodes
6(b)
1. Identify the number of choices for the 3 identical letters and determine the available pool for the remaining characters.
The letters in "venessa see" have the following frequencies: v(1), e(4), n(1), s(3), a(1).
To have exactly 3 identical letters, the chosen letter must appear at least 3 times in the name.
Thus, there are choices for this letter ('e' or 's').
Once the identical letters are chosen, we need 4 more distinct characters to form a 7-character passcode.
The pool of available distinct characters consists of the remaining distinct letters and the digits (1 to 9).
Total characters in the remaining pool .
2. Calculate the number of valid ways to select the remaining 4 distinct characters, ensuring at least one digit is included.
We need to select 4 distinct characters from the 13 available.
Total unrestricted ways to select 4 characters .
The passcode must be alpha-numeric. Since we already have 3 letters, we must ensure at least 1 digit is included. The only invalid selection is choosing 4 letters (0 digits).
Number of ways to select 4 letters from the 4 available letters .
Valid selections for the 4 distinct characters .
3. Determine the number of arrangements for the 7 characters and calculate the total number of passcodes.
For any chosen set of 7 characters (which consists of 3 identical letters and 4 distinct characters), the number of distinguishable arrangements is .
Total number of passcodes
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