Oxnard Casualty wants to ensure that their e-mail server has 99.98 percent reliability. They will use several independent servers in parallel, each of which is 95 percent reliable. What is the smallest number of independent file servers that will accomplish the goal?
A)1
B)2
C)3
D)4

Question

Quizplus · Accepted Answer

Let's assume there are 'n' independent file servers used for parallel processing. 
The probability of all 'n' servers failing simultaneously can be calculated as (0.05)^n. 
Therefore, the probability of at least one server out of 'n' servers functioning properly can be given as 1 - (0.05)^n.
As we want to achieve 99.98% reliability, the probability of server failure should be less than or equal to 0.0002. 
So, we need to find the minimum value of 'n' such that 1 - (0.05)^n <=0.0002.

By solving the above equation, we get n = 3. Therefore, at least 3 independent file servers are required to achieve a reliability of 99.98%.

Oxnard Casualty Wants to Ensure That Their E-Mail Server Has