Question

Alfred wants to become a member of an online discussion forum. The website requires him to create a password. The password must consist of 5 characters, of which 3 must be letters and 2 must be digits. The first character of the password must be a letter. Also, repetition of the same letter in the password is not permitted but repetition of digits is permitted. How many possible ways can Alfred create the password?

So I know the answer is 9,360,000 but I got 18,720,000. This is because I thought the partition is 4! / (2! * 1! * 1!) but apparently the partition is 4! / (2! * 2!) and this confuses me. I thought that since the letters can't be repeated, we'd put one unique letter in one group, the other unique letter in the other group, and the two repeated digits in the last group. Can somebody explain this to me?

Answer 1

Alfred can create the password in 9,360,000 possible ways

Explanation:

Given the data in the questions;

Password of five characters must contain; three letters and 2 digits

[][][][][]

Now, using the five boxes above which as to be filled with three letters and two digits, such that; The first character of the password must be a letter and repetition of the same letter in the password is not allowed while repetition of digits is allowed.

Now,

we know that there are 26 alphabets in English language, so the first box can be filled in 26 ways { must be a letter }

next four will be filed with two letters and two digits

so two boxes from the four can be chosen in the following ways;

⇒ ⁴C₂ = 4! / ( 2!(4-2)! = 24 / 4 = 6 { ways }

now for each choice, we can fill the chosen two boxes with letters in;

⇒ 25 × 24 = 600 { ways}

and the next two by digits; ( 0, 1, 2, ..... 9)

⇒ 10 × 10 = 100 { ways }

So in total, the possible number of ways he can create a password is;

⇒ 26 × 6 × 600 × 100

= 9,360,000 possible ways

Therefore, Alfred can create the password in 9,360,000 possible ways