Solution: 2014 Winter Final - 2

Author: Michiel Smid

Question

A password consists of 10 characters, each character being a lowercase letter or a digit. A password must contain at least one digit and at most three digits. How many passwords are there?

(a)

none of the above

(b)

${10 \choose 1}26^9 + {10 \choose 2}26^8 + {10 \choose 3}26^7$

(c)

${10 \choose 1} \cdot 10 \cdot 26^9 + {10 \choose 2} \cdot 10^2 \cdot 26^8 +$ $ {10 \choose 3} \cdot 10^3 \cdot 26^7$

(d)

$10 \cdot 26^9 + 10^2 \cdot 26^8 + 10^3 \cdot 26^7$

Solution

Let $ A $ be the set of passwords with one digit
There are $ 10 $ ways to choose the position of the digit: 10
The digit has $ 10 $ possible outcomes $ ( { 0,1,2,3,4,5,6,7,8,9 } ) $: 10
The remaining 9 characters each have 26 possible outcomes
$ ( { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z } )$: $26^9 $
$ |A| = 10 \times 10 \times 26^9 $
Let $ B $ be the set of passwords with two digits
There are 10 positions to choose 2 digits: $ \binom{10}{2} $
The two digits combined have $ 10 \times 10 $ possible outcomes: $10^2$
The remaining 8 characters each have 26 possible outcomes: $ 26^8 $
$ |B| = \binom{10}{2} \times 10^2 \times 26^8 $
Let $ C $ be the set of passwords with three digits
There are 10 positions to choose 3 digits: $ \binom{10}{3} $
The three digits have $ 10 \times 10 \times 10 $ possible outcomes: $10^3$
The remaining 7 characters each have 26 possible outcomes: $ 26^7 $
$ |C| = \binom{10}{3} \times {10}^3 \times 26^7 $

The total number of passwords is the sum of the passwords in sets $ A $, $ B $, or $ C $:

$ |A| + |B| + |C| $

$ = \binom{10}{1} \times 10 \times 26^9 + \binom{10}{2} \times 10^2 \times 26^8 + \binom{10}{3} \times 10^3 \times 26^7 $