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The availability of a complex software is 90%. Its Mean Time Between Failure (MTBF) is 200 days. Because of the critical nature of the usage, the organization deploying the software further enhanced it to obtain an availability of 95%. In the process, the Mean Time To Repair (MTTR) increased by 5 days. What is the MTBF of the enhanced software?

1. 205 days
2. 300 days
3. 500 days
4. 700 days

Availability = MTBF/(MTBF + MTTR)

Case 1 :

0.9 = 200/(200 + x)

x = 22.22

Case 2 :

0.95 = y/(y+22.22+5)

0.95 = y/(y+27.22)

y = 517.18 (It is near to option C)

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Availability is given by $\frac{Mean Time To Failure}{MeanTimeToFailure+MeanTimeToRepair}$

And $Mean Time Between Failure = Mean Time To Failure + Mean Time To Repair$

Before Enhancement:

Given 90 = $\frac{MTTF}{200}$

MTTF  = 180 days

MTTR = MTBF – MTTF

MTTR = 20 days

After Enhancement :

MTTR increases by 5 days. So MTTR = 25 days

95 % = $\frac{MTTF}{MTTF+25}$

MTTF = 475

MTBF = 475 + 25

MTBF = 500 exactly

Option (c)
Given,

Availability = 90%
MTBF = 200 Days=200*24=4800 Hours.

As we know that,

Availability = MTBF*100/MTBF+MTTR

90 = 4800*100/4800+MTTR

432000+90MTTR = 480000

90MTTR = 480000-432000

90MTTR = 48000

MTTR = 48000/90

MTTR = 533.33 Hours.

According to question, we have to find MTBF,

It is given that, MTTR is increased by 5 Days = 5*24=120 hours.

Availability = MTBF*100/MTBF+MTTR

95 = MTBF*100/MTBF+533.33+120

95 = 100MTBF/MTBF+653.33

95MTBF + 62066.35 = 100MTBF

62066.35 = 100MTBF – 95MTBF

62066.35 = 5MTBF

62066.35/5 = MTBF

MTBF = 12413.27 Hours

Convert it into Days,

MTBF = 12413.27/24

MTBF = 517.21 Days

So, Option C is the Correct Answer.