Question from GRE Computer Science practice test

[ambreen_cs]

1. Suppose that a certain software product has a mean time between failures of 10,000 hours and has a mean time

to repair of 20 hours. If the product is used by 100 customers, what is its availability?

(A) 80% (B) 90% (C) 98% (D) 99.8% (E) 100%

Someone please explain this.

[e(ho0n3]

You can solve this in two ways: (1) Look up the availability formula in your software engineering book and just plug in the numbers or (2) use some simple reasoning and find the solution independently.

Suppose you want to solve this problem using method (2). The 'usage' cycle for this software lasts 10,020 hours. How many hours is the software available during this cycle?

[Suhaib1]

The answer is E.
10,000/10,000+20= 99.8%

[e(ho0n3]

The answer is D and 10,000/10,000 + 20 = 21. I think what you meant was 10000/(10000 + 20) = 99.8%.

[Suhaib1]

Yes, the answer is D-99.8%. Also I am a little careless about putting brackets, but its obvious from the answer and its not a very complex calculataion either, so I didnt bother putting them.

Suhaib