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.

  1. (a)

    Find the number of different passcodes if all the letters and numbers are distinct such that

    1. (i)

      they begin with a letter and end with a digit.

    2. (ii)

      all the letters and digits are to be separated.

  2. (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": {v,e,n,s,a}\{v, e, n, s, a\} (5 letters)
Distinct digits from 1 to 9: {1,2,3,4,5,6,7,8,9}\{1, 2, 3, 4, 5, 6, 7, 8, 9\} (9 digits)
Total distinct characters available = 5+9=145 + 9 = 14

2. Determine the number of choices for the restricted first and last positions.

First position (must be a letter): 55 choices
Last position (must be a digit): 99 choices
Number of characters remaining after filling the first and last positions = 14−2=1214 - 2 = 12

3. Determine the number of ways to arrange characters in the remaining positions and calculate the total number of passcodes.

Remaining positions to fill = 7−2=57 - 2 = 5
Ways to arrange the remaining 12 characters into 5 positions = 12P5=95,040^{12}P_5 = 95,040
Total number of passcodes = 5×9×12P55 \times 9 \times {}^{12}P_5
=45×95,040=4,276,800= 45 \times 95,040 = 4,276,800

Answer

4,276,8004,276,800

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:

  1. L-D-L-D-L-D-L (4 letters, 3 digits)
  2. 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 55 distinct letters (v, e, n, s, a) and 99 distinct digits.
Choosing and arranging 4 letters and 3 digits:
Ways=5P4×9P3\text{Ways} = {}^5P_4 \times {}^9P_3
=120×504=60480= 120 \times 504 = 60480

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:
Ways=5P3×9P4\text{Ways} = {}^5P_3 \times {}^9P_4
=60×3024=181440= 60 \times 3024 = 181440

Total number of passcodes =60480+181440=241920= 60480 + 181440 = 241920

Answer

241920241920

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 (21)=2\binom{2}{1} = 2 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 44 distinct letters and the 99 digits (1 to 9).
Total characters in the remaining pool =4+9=13= 4 + 9 = 13.

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 =(134)=715= \binom{13}{4} = 715.

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 =(44)=1= \binom{4}{4} = 1.

Valid selections for the 4 distinct characters =715−1=714= 715 - 1 = 714.

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 7!3!\frac{7!}{3!}.

Total number of passcodes =2×714×7!3!= 2 \times 714 \times \frac{7!}{3!}
=1428×840= 1428 \times 840
=1,199,520= 1,199,520

Answer

1,199,5201,199,520

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