MATH SOLVE

3 months ago

Q:
# The secretary in Exercises 2.121 and 3.16 was given n computer passwords and tries the passwords at random. Exactly one of the passwords permits access to a computer ﬁle. Suppose now that the secretary selects a password, tries it, and—if it does not work—puts it back in with the other passwords before randomly selecting the next password to try (not a very clever secretary!). What is the probability that the correct password is found on the sixth try?

Accepted Solution

A:

Answer:There is a [tex]\frac{(n-1)^{5}}{n^{6}}[/tex] probability that the correct password is found on the sixth try.Step-by-step explanation:We have n passwords, only 1 is correct.So,Since a password is put back with other, at each try, we have that:The probability that a password is correct is [tex]\frac{1}{n}[/tex]The probability that a password is incorrect is [tex]\frac{n-1}{n}[/tex].There are 6 tries.We want the first five to be wrong. So each one of the first five tries has a probability of [tex]\frac{n-1}{n}[/tex]. So, for the first five tries, the probability of getting the desired outcome is [tex]\frac{(n-1)^{5}}{n^{5}}[/tex].We want to get it right at the sixth try. The probability of sixth try being correct is [tex]\frac{1}{n}[/tex].So, the probability that the first five tries are wrong AND the sixth is correct is:[tex]P = \frac{(n-1)^{5}}{n^{5}}\frac{1}{n} = \frac{(n-1)^{5}}{n^{6}}[/tex]There is a [tex]\frac{(n-1)^{5}}{n^{6}}[/tex] probability that the correct password is found on the sixth try.